You can still turn processor nap on and off with the newest version of OS X (10.5.7 as of this writing) and the newest version of the CHUD tools (4.6.2 as of this writing), which are installed with Xcode. You simply have to install the Processor System Preferences pane manually after installing the CHUD tools. I did this on my old dual G5 2.0 GHz to eliminate the CPU's chirping sound. Here's how...
Once C.H.U.D. tools is installed, in Finder go to /Developer » Extras » PreferencePanes. Drag the Processor.prefpane file to
/System » Library » PreferencePanes; the Finder will ask you to authenticate as an administrator to put it there, and then you're done!
You might have to reboot, but the Processor panel will then be available in System Preferences to turn nap on or off. Hope this helps you out!
[robg adds: This was covered originally in this hint, but it seems things have changed -- the Processor System Preferences panel used to be automatically installed.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090630204454402