I tried the earlier version of SetCPU (1.5.4) with the Nexus One settings and it really didn't feel like it was doing anything. After that I tried Overclock Widget and messed around with that for a couple of days and while it helped, it still didn't give me the results that others say they are getting.
The newest version of SetCPU automatically detects your cpu if you choose that setting and since installing this app, I have noticed an increase in battery performance.
Check out the app in the market if you want (especially if you want to donate as this app is AWESOME!) Or follow this link to the xda developers thread and download there.
I haven't used this app for more than an hour so hopefully this isn't a placebo, but it sure doesn't hurt to try it!
http://forum.xda-developers.com/showthread.php?t=505419
Edit:
You must be rooted to use this app and in order to even see this in the market.