Ok here's what i found out.. when connected via bluetooth or earbuds... it locks screen... now while on a normal call without being connected that way, it acts as a end call button.... tried on 3gs, and 4 at apple with 4.0 or 4.0.1
it was one of those things that happened while trying to remove bumper for one set of ear buds i have since the shield is bigger than the bumper allows and i was connected to a call... normally i have buds or bluetooth connected.. 3g and 3gs i never noticed since there was no bumper hence I never used the sleep button during a call lol