My screen won't flip on my iphone4. There are a couple of games that it flips for, but it doesn't flip for anything else, not safari, not mail, not photos. I've tried powering off completely to no avail.
If you are talking about the screen rotation, double click on home button to open up the multi-tasking bar and scroll left, you should see a round arrow with a lock symbol on it and see if that is locked or not.