Comment 2 for bug 833841

I have the same problem on Natty. The root cause seems to be this line:

setting up libgl1-mesa-glx (7.10.2-0ubuntu2.1) ...
update-alternatives: error: alternative link /usr/lib/xorg/extra-modules is already managed by i386-linux-gnu_gl_conf.
dpkg: error processing libgl1-mesa-glx (--configure):
 subprocess installed post-installation script returned error exit status 2

My guess is that this is because of the Nvidia driver because of the content of /usr/lib/xorg/extra-modules:

$ ll /usr/lib/xorg/extra-modules
lrwxrwxrwx 1 root root 53 2011-08-08 09:30 /usr/lib/xorg/extra-modules -> /etc/alternatives/x86_64-linux-gnu_xorg_extra_modules/
$ ll /etc/alternatives/x86_64-linux-gnu_xorg_extra_modules/
total 12960
drwxr-xr-x 2 root root 4096 2011-08-08 09:30 ./
drwxr-xr-x 6 root root 4096 2011-08-08 09:30 ../
lrwxrwxrwx 1 root root 16 2011-08-08 09:30 libglx.so -> libglx.so.280.13
-rw-r--r-- 1 root root 6356552 2011-08-01 20:23 libglx.so.280.13
-rw-r--r-- 1 root root 6901792 2011-08-01 20:23 nvidia_drv.so