Just got back from the Apple Store to exchange my cracking white iPhone. Simple process. I showed the genius the cracking, he said they are aware of the problem and official policy right now is to replace the phone under warranty. They don't know why the phones are cracking though, so he couldn't guarentee a replacement wouldn't crack, too. He asked me some questions and filled out a survey, since they are trying to find a pattern. I think they're trying to blame it on being dropped, or being put in cases that are too tight, but I've never dropped the phone and don't use a case. He did definitely give me a refurbished phone even though I had purchased my phone from Apple, but said that one thing all the refurbished phones get, regardless of the repair they came in for, was a replacement of the back plastic casing. So I traded my cracked phone with a badly fitted bezzle and screen for a phone that now has the crooked sleep button and a tough silent ringer switch, but I think I came out ahead. A crooked sleep button is something I'll get used to, the cracking was only going to get worse.
Hope this helps some people who are on the fence about getting their phones replaced.