First off, it's likely the rear derailleur hanger that is bent and not the RD itself. There is a Park Tool that indicates how off the hanger is and allows you to bend it back by hand: http://www.parktool.com/repair/readhowto.asp?id=39

Most aluminum frames come with removable hangers that are meant to be replaced if bent: http://www.sheldonbrown.com/harris/hangers/

If it's really the RD, I'd just replace it.