Just tell them your story. From what I've heard, they're pretty understanding about that type of stuff and will give you a new one.
I actually have scheduled an appointment for Monday to try and get myself a new one. I've been using mine since July, but just recently it's started locking up randomly when I try to unlock it, and sometimes I'll get text messages that show up in the blue alert box, but then don't appear in the conversation. The most annoying thing of it all, though, is that the speaker volume is between 1/2 and 3/4 as loud as on some of my friends' iPhones.