It's clear that Apple doesn't want folks fooling around with their iPhone's physical hardware, as the recently dubbed 'Pentalobular' screws that may rest on the bottom of your iPhone 4 prove.
Recently, Apple has switched over to an all-new screw set for the iPhone 4, similar to the screws that grace the battery covers of the older MacBook Pros, and current MacBook Air.
If you've owned an iPhone 4 for quite some time, then chances are you still have the normal Phillips screw set, but if you've just stepped into the game, you might want to give this a read...