The risk for SIDS significantly decreases when your lo is able to roll over
The risk doesn't go away until after 1 year, but part of the thinking behind SIDS is that your lo buries his/her head into the matress, and the auto mechanism in the brain that wakes you up if you're suffocating doesn't kick in. Usually if your lo can roll over and has mastered head control the risk decreases.
Mine has been sleeping on her tummy since about 3 months. I was incredibly worried at first, but now I'm not so worried. She does sometimes move her face flat down into the cot while sleeping, which does tend to freak me out sometimes, but she moves it back to the side then after about 30 seconds. As long as you don't have a lot of covers and such around for her to bury herself in, you should be ok.
If it really concerns you, I would wait until she's asleep and in deep sleep before rolling her back over. That way it doesn't eventually turn into a game.
*HUGS*
Sonya =P