Usually if you have an incompatible timecounter set then the logs get filled with errors very quickly. Something like:
kernel: timecounter TSC must not be in use when changing frequencies; change denied
You can check from the command line:
[2.0.1-RELEASE][root@pfsense.fire.box]/root(3): sysctl kern.timecounter.hardware
kern.timecounter.hardware: TSC
[2.0.1-RELEASE][root@pfsense.fire.box]/root(4): sysctl kern.timecounter.choice
kern.timecounter.choice: TSC(800) i8254(0) dummy(-1000000)
I don't have speedstep on that machine so I haven't switched from TSC.
I think I noted most of what you need to do here:
http://forum.pfsense.org/index.php/topic,20095.msg161139.html#msg161139
My experience with 2GHz Pentium-M 760 was that the est(4) driver brought up an incorrect set of speedstep frequencies and the logs filled with 'calcru: runtime went backwards' errors. This may be solvable though.
On my box I saved 5W at idle, about 15%. I saved far more with a DC power supply.
Steve