Do you have a second battery? Maybe the one you have has bad contacts or something. I know my XSi starts acting weird when the battery is wearing down...and the battery meter sometimes still shows "full charge", but in reality the battery is nearly-dead.
I think, before I'd send it back in, I'd get a battery at a local camera shop (you can always return it if you find it's not the problem) and try it out with a full charge. Also, when you sent it to Canon, did you send your battery? Maybe they used their own battery and found no problems. Did the "work order" thing they send out with warranty repairs say they did anything to your camera, or just found it "in spec"?