Why pink? Well, survey says pink has a calming effect in jail. (Our Sheriff Joe A. makes all inmates wear pink underwear). So I guess the pink battery is supposed to deter would-be-jailbreakers. Get it? pink/jail/jailbreaking... Sorry, that joke needs some work.
It is also for using the phone in bright sunlight. This is a feature exclusive to the 3GS I believe. Wish we could change the colors of all the parts of the device (black background, grey bar), text on top) whatever colors we wanted. That would be a REALLY EASY thing to do and a lot of people would like it.