I have the same problem. I had to get my battery replaced because it was draining an an unbelievable rate, but the problem remains. The suggestion was to do a clean system install, but that has not fixed the problem either.
I am thinking that there is a firmware or other hardware problem that is keeping the phone from going to sleep. It has been suggested that this is a non-fixable problem and that I need to get the phone replaced. I hate having to do that, but unless Apple recognizes the problem and there is a software fix available, that will be my only choice, because the battery does not get the life it should.