SetCPU has received a very, very nice update, where the user interface has been updated to rock along with Ice Cream Sandwich's look. The version 2.3.0 update also sports a frequency slider, which is a lot easier for your finger, and device.
Also included is a governor menu for adjusting processor scaling, added support for multi-core devices, as well as the usual bug fixes. All of it works on rooted Android devices, including Galaxy Nexus, Nexus S, Motorola Droid, Nexus One, G1, G2, Samsung Galaxy S II, myTouch 4G, and Motorola Xoom.
Overclocking your device would be great, but underclocking is also a good start to more battery life. SetCPU can also make good use of profiles, and change CPU speeds when certain conditions arise. This is great for when the phone is asleep, or charging, when the battery level drops below a certain point, when the phone's temperature reaches a certain point, or during specific times of the day.
I just might root my GS II and try this myself!