Replace, replace, replace! I had cracks in my phone, got it replaced! No problems at all. My 3G chip was fried, got it replaced in fact the very day i found out that it was fried! The tech on the phone insisted i go immediately if not sooner to the closest Apple store and have it replaced. He was adamant about it, he made the appointment for me with the Genius and I'm glad I replaced it. (You know, in order to have a working phone, I needed it replaced.)
Apple is good about replacing the phone when there are defects.