After a year you need to pay the out of warranty replacement cost which I think is $150 or something. Doesn't hurt to go in to an Apple Store and see if they'll do it for free. They have been known for going above and beyond their requirements for customer care on occasion, although not so much since the 4S came out. You also might want to just disassemble part of the phone and maybe clean out the button area. I've been having the same problem, and so has a friend of mine. If it's a mechanical issue it should be easy to fix on your own. To me it seems like you need to press the home button 2 or 3 times for it to register properly. Contributes to the overall feeling of lagginess, and part of the reason I switched back to my 4S.
EDIT: Also found this
. Summary: tap the bottom of the phone while applying a vacuum to the dock connector port. Seems to have helped a lot of people.