Sorry to hijack your thread, but, I have a question regarding this matter as well.
My DS is a back sleeper but we have had a lot of trouble recently with his sleeping (2 hourly waking) after reading the lovely ladies replies here I am thinking maybe I should attempt getting him to sleep on his tummy. At the moment I am trying to get him to sleep without being wrapped so I have him in a gro-bag (sleeping bag), do you think he would be able to sleep on his tummy in a gro-bag??