I recently installed a small free utility called Cee Pee You on my MacBook Pro that posts your CPU usage in the menu bar. Much to my surprise, my CPU usage was very high, more often then not pegging at over 90%, regardless of what I was doing -- idle or not.
Confused, intrigued and a bit nervous, I opened Terminal and opened a process list -- ps aux --- to see what the culrpit was. To my surprise, I had a printmanager job running and hogging the CPU, even though I was not currently printing anything. To delete the print job, I opened Printer Setup Utility, looked at my list of printers, and found that a print job that I had accidently sent to my Canon i960 while at work instead of to the Xerox printer was the culprit. I deleted the print job, and the CPU monitor immediately dropped to low numbers, under 20%, almost all of the time.
The lessons I learned from this that might be helpful are:
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006082723260630