Practically all the babies in my mothers group had flat heads at the back, as we all slept our babies on their backs. As they grow older, their heads naturally go back to a normal shape. The SIDS awareness people don't recommend putting one of those padded things in the cot to keep baby on his or her side. I think that in a few months your baby's head will be fine. Eventually they start to move around in their sleep, and even end up on their tummy. By the time they start doing this on their own, it is apparently safe for them, but I wouldn't do it on purpose.