I still have this problem (hard freezing) with Dapper with a Intel Centrino 1800.
While powernowd is installed it always freezes after some minutes.
I have following versions:
powernowd 0.97-1ubuntu1
linux-image-2.6.15-23-686 (2.6.15-23-39)

I have also tested an earlyer version 0.96 for the developers homepage... same problem... also tested with other Modes e.g. AGGRESSIVE... same...

I first thought that it was something with the ati drivers... but it freezed with fglrx, ati, vesa and the official fglrx...

Then I have increased the verbosity of powernowd and got following output in the syslog:
May 28 17:57:27 localhost powernowd: Using PASSIVE mode. 
May 28 17:57:27 localhost powernowd: PowerNow Daemon v0.97, (c) 2003-2006 John Clemens 
May 28 17:57:27 localhost powernowd: Settings: 
May 28 17:57:27 localhost powernowd:   verbosity:        6 
May 28 17:57:27 localhost powernowd:   mode:             2     (PASSIVE) 
May 28 17:57:27 localhost powernowd:   step:           100 MHz (100000 kHz) 
May 28 17:57:27 localhost powernowd:   lowwater:        20 % 
May 28 17:57:27 localhost powernowd:   highwater:       80 % 
May 28 17:57:27 localhost powernowd:   poll interval: 1000 ms 
May 28 17:57:27 localhost powernowd: Found 1 scalable unit:  -- 1 'CPU' per scalable unit 
May 28 17:57:27 localhost powernowd:   cpu0: 600Mhz - 1800Mhz (7 steps) 
May 28 17:57:27 localhost powernowd:      step1 : 1800Mhz 
May 28 17:57:27 localhost powernowd:      step2 : 1600Mhz 
May 28 17:57:27 localhost powernowd:      step3 : 1400Mhz 
May 28 17:57:27 localhost powernowd:      step4 : 1200Mhz 
May 28 17:57:27 localhost powernowd:      step5 : 1000Mhz 
May 28 17:57:27 localhost powernowd:      step6 : 800Mhz 
May 28 17:57:27 localhost powernowd:      step7 : 600Mhz 
May 28 17:57:28 localhost powernowd: i = 0, cpubase = 0, 
May 28 17:57:28 localhost powernowd: PCT = 0.019608 
May 28 17:57:28 localhost powernowd: got here LOWER 
May 28 17:57:28 localhost powernowd: change = 0, change2 = 0 
May 28 17:57:28 localhost powernowd: Setting speed to 600000 
May 28 17:57:28 localhost powernowd: mode=0, str=600000 
May 28 17:57:29 localhost powernowd: i = 0, cpubase = 0, 
...
many more of those...
...
May 28 18:02:26 localhost powernowd: change = 0, change2 = 1 
May 28 18:02:27 localhost powernowd: i = 0, cpubase = 0, 
May 28 18:02:27 localhost powernowd: PCT = 0.010000 
May 28 18:02:27 localhost powernowd: change = 0, change2 = 1 
May 28 18:02:28 localhost powernowd: i = 0, cpubase = 0, 
May 28 18:02:28 localhost powernowd: PCT = 0.000000 
May 28 18:02:28 localhost powernowd: change = 0, change2 = 1 
May 28 20:03:47 localhost syslogd 1.4.1#17ubuntu7: restart.


About my Processor:
cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 13
model name      : Intel(R) Pentium(R) M processor 1.80GHz
stepping        : 6
cpu MHz         : 1792.961
cache size      : 2048 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr mce cx8 sep mtrr pge mca cmov pat clflush dts acpi mmx fxsr sse sse2 ss tm pbe est tm2
bogomips        : 3587.46