Don't be freaked out. While it's easier for babies to roll onto their belly than back again, there are so many other factors in SIDS deaths than just putting baby down on their belly. Make sure their sleeping space is firm. Place them to sleep on their back in the evening and after nursing, if that makes you feel better, but don't stress about what the baby does for herself.
I don't know any links or better, more helpful information on this, though. Anyone?
"She is a mermaid, but approach her with caution. Her mind swims at a depth most would drown in."