My DS has just taken to sleeping on his tummy. I haven't really worried about it, but he is just getting over the flu, a bad one & I'm glad he likes his tummy, coz he would've choked had I put him on his back.
So while he was sick I made sure he was on his tummy as it was safest at the time. I put him on his side or back now, but he's usually on his belly by morning.

Takes after his mummy I guess!