I read in "What to Expect in the First Year" that you should still put babies to bed on their backs until the first birthday but if they roll onto their fronts - then not to worry. The book advises 'that a baby who is able to change positions easily is at a significantly decreased risk for SIDS.' This is for 2 reasons:
1) The high risk period for SIDS is generally passed by the time a baby can turn over, and
2) A baby who can flip is better equipped to protect herself from whatever it is about tummy sleeping that increases SIDS risk.
Hopefully, this info will give some peace of mind.