As described on Wikipedia, SpeedStep "is a trademark for a series of dynamic frequency scaling technologies (including SpeedStep, SpeedStep II, and SpeedStep III) built into some Intel microprocessors that allow the clock speed of the processor to be dynamically changed by software. This allows the processor to meet the instantaneous performance needs of the operation being performed, while minimizing power draw and heat dissipation."
Apple implements SpeedStep on certain Macs, but not all of them (and if you've built your own Mac, you may not have any SpeedStep support). Here's how to enable SpeedStep on any Mac: Use the xnu speedstep kernel extension. Just download IntelEnhancedSpeedStep.kext.1.4.5.zip, then copy IntelEnhancedSpeedStep.kext into /System/Library/Extensions. Don`t forget to repair permission (in Disk Utility) when done, then reboot your Mac.
Now we can see dynamic frequency and power changes. Also see sysctl -a | grep kern.cputhrottle from Terminal, and read the AutoThrottle wiki for more information. There you'll find the command sysctl -w kern.cputhrottle_targetload=XY. However, you can also edit the TargetCPULoad parameter in /System » Library » Extensions » IntelEnhancedSpeedStep.kext » Contents » Info.plist file.
[robg adds: I haven't tested this one, and would categorize this hint as experimental -- use at your own risk!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090309054944734