I had some nasty burn in on my focus. You could see a lot of the letters from the keyboard (specifically the smiley face and paper clip). A couple names of people I text could be seen in the top left corner. The giant "People" in the people hub was starting to burn in as well, along with the text boxes for text messages.
So, I brought my phone into AT&T. The man helping us (Chris) instantly noticed the burn in, especially where there were white screens. He put us on the phone with the Customer Service center and we explained the problem. First they claimed that the phone was out of warranty (Chris reminded me that the phone hadn't even been out a year yet) and then they claimed that it was my fault. Chris took the phone, tried to explain burn in and told them that a Microsoft Rep had come into their store last week to explain the problem (I don't know how valid that is, but I guess it is possible). The woman on the phone still wasn't going to do anything about it so we hung up. Chris brought his manager over and they both agreed to replace my old phone and just give me a new one. I was VERY pleased with the service there and I'm glad I don't have to live with a phone with some heavy screen burn in (Not to mention I only had the phone for eight months)
So in short, if you are having issues of burn in on your phone, try taking it into the AT&T store and see what they can do to help. The call center was frustrating to deal with, but the way the store handled it was top notch customer service if you ask me.