Yeah ultimatly doing nothing in most cases.
In fact, if you have a quadcore or better, you can uc your CPU to ~1,5 Ghz while still having constant 60fps as long as your GPU can handle it.
some kind of proof made moments ago:
3,2 Ghz:
http://img694.imageshack.us/img694/9871/32ghzo.jpg
2,1 Ghz:
http://img143.imageshack.us/img143/8621/21ghz.jpg
2 Instances of WoW,
both at 1280x960 windowed with 2xAA and Ultra settings.
Foreground over SW.
Background botting somewhere. Yes i bot chars to 85, problem? World is dead anyway...
In adition, Skype, Frirefox and the bot is running.
Both have 60FPS on a HD 4870 nonoc, with a nonoc and uc 955 BE.
WoW CPU intense? Order a crane and try to get out this old thing you call PC!
There are only a few games out there, that really use your CPU. Mainly the ones that use crazy ammounts of physics, REAL PHYSICS, not the scripted ones.