My iPhone4 is coming apart. I've had it for 2 years. Can I get it replaced?

I'm less concerned about getting an upgrade and more concerned with the chances of it one day just not working. It's my primary phone and even my mother needs to use it sometime. What do I need to do to get it replaced? How much would it cost?

It's not a simple case of a cracked screen. The casing literally separating, probably from overheating, and I've noticed that it's opened up even more today.