Sam loves sleeping on his front, and if he is really unsettled for any reason and he's due a nap,we always put him down that way and he falls asleep almost instantly. That said, he has been able to turn over and back again from 3 weeks old (yes, 3 WEEKS - hv thought I was a nutter until she saw it for herself, but it turned out this was because he has some back tension issues so rolls over to get out of pain).
I put him to bed on his back but don't roll him back if he has gone onto his front.
Lisa xxx