I know that my phone -- HTC's Evo -- actually has a diagnostic tool thats sole purpose is to drain the battery as fast as possible. Open up your dialer and enter:
You don't need to press send or anything. A recent update for the Evo blocked these tools so hopefully you can still access them. You can also look up device specific codes if yours happens to be different. Rooted users have found a way to get these tools back.
The utility turns up brightness all the way, prevents sleep, turns on the camera and flash, turns on the GPS and constantly polls it, etc. Basically everything that can drain the battery but not damage the phone is done (ie it doesn't constantly vibrate because that would probably be bad).