Using the tool nvidia-xconfig I was able to reactivate the nVidia driver in Xorg. But after rebooting the tool nvidia-settings still complained:
"You do not appear to be using the NVIDIA X driver.  Please edit your X configuration file (just run `nvidia-xconfig` as root), and restart the X server."

After that I tried to enable a driver using Jockey and selected the first driver without success (could not activate the driver). Jockey log says:

--2012-03-08 19:18:11,635 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 19:18:11,858 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 19:18:12,015 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 19:18:12,020 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 19:18:12,036 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 19:18:12,065 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 19:18:12,085 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:18:12,183 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:18:12,209 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:18:12,229 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 19:18:13,188 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:18:13,237 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:18:13,283 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:18:32,447 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:18:32,447 ERROR: XorgDriverHandler.enable(): package or module not installed, aborting
2012-03-08 19:19:38,193 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 19:19:38,284 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 19:19:38,348 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 19:19:38,354 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 19:19:38,382 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 19:19:38,432 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 19:19:38,489 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:19:38,665 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:19:38,720 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:19:38,774 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 19:19:39,477 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:19:39,538 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:19:39,598 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:45:20,125 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 19:45:20,317 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 19:45:20,451 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 19:45:20,455 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 19:45:20,471 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 19:45:20,512 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 19:45:20,533 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:45:20,697 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:45:20,754 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:45:20,811 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 19:45:21,686 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:45:21,746 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:45:21,805 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:45:41,320 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:45:41,321 ERROR: XorgDriverHandler.enable(): package or module not installed, aborting
2012-03-08 19:48:20,752 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 19:48:20,852 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 19:48:20,908 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 19:48:20,912 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 19:48:20,928 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 19:48:20,946 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 19:48:20,975 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:48:21,051 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:48:21,073 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:48:21,102 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 19:48:22,209 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:48:22,311 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:48:22,435 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:48:37,443 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 19:48:37,536 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 19:48:37,603 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 19:48:37,607 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 19:48:37,624 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 19:48:37,645 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 19:48:37,671 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:48:37,746 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:48:37,771 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 19:48:37,799 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 19:48:38,908 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 19:48:38,959 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 19:48:39,009 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 20:22:01,395 WARNING: modinfo for module fglrx_updates failed: ERROR: modinfo: could not find module fglrx_updates
2012-03-08 20:22:01,493 WARNING: modinfo for module fglrx failed: ERROR: modinfo: could not find module fglrx
2012-03-08 20:22:01,557 WARNING: modinfo for module ath_pci failed: ERROR: modinfo: could not find module ath_pci
2012-03-08 20:22:01,561 WARNING: modinfo for module vmxnet failed: ERROR: modinfo: could not find module vmxnet
2012-03-08 20:22:01,577 WARNING: modinfo for module wl failed: ERROR: modinfo: could not find module wl
2012-03-08 20:22:01,600 WARNING: modinfo for module nvidia_96 failed: ERROR: modinfo: could not find module nvidia_96
2012-03-08 20:22:01,625 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 20:22:01,709 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 20:22:01,732 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 20:22:01,759 WARNING: modinfo for module nvidia_96_updates failed: ERROR: modinfo: could not find module nvidia_96_updates
2012-03-08 20:22:02,891 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 20:22:02,944 WARNING: modinfo for module nvidia_173_updates failed: ERROR: modinfo: could not find module nvidia_173_updates
2012-03-08 20:22:02,999 WARNING: modinfo for module nvidia_173 failed: ERROR: modinfo: could not find module nvidia_173
2012-03-08 20:23:53,794 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-03-08 20:23:53,794 ERROR: XorgDriverHandler.enable(): package or module not installed, aborting
--

But choosing the second nVidia driver appeared to be successful. However the tool nvidia-settings still complains that there is no  nVidia driver and also the tool nvidia-detector returns "none".

However, the tool nvidia-smi returns:

# sudo nvidia-smi
Thu Mar  8 20:33:56 2012       
+------------------------------------------------------+                       
| NVIDIA-SMI 3.295.20   Driver Version: 295.20         |                       
|-------------------------------+----------------------+----------------------+
| Nb.  Name                     | Bus Id        Disp.  | Volatile ECC SB / DB |
| Fan   Temp   Power Usage /Cap | Memory Usage         | GPU Util. Compute M. |
|===============================+======================+======================|
| 0.  GeForce 9600M GS          | 0000:01:00.0  N/A    |       N/A        N/A |
|  N/A   68 C  N/A   N/A /  N/A |   0%    3MB / 1023MB |  N/A      Default    |
|-------------------------------+----------------------+----------------------|
| Compute processes:                                               GPU Memory |
|  GPU  PID     Process name                                       Usage      |
|=============================================================================|
|  0.           Not Supported                                                 |
+-----------------------------------------------------------------------------+

I hope this helps in determining the problem.