I think if a baby can roll themselves over onto their tummy then they are just as capable of rolling back if necessary. Tummy sleeping is really more a problem with younger babies who don't have much strength or muscle control and may not be able to move themselves if they need to.

I've seen those wedge things and one lady in my mother's group was talked into buying a device that kept the baby in place in bed at night by means of a velcro strap (a bit like a seat belt). All I know is that the SIDS people don't endorse any of these products - not because they are against people selling baby products but more because they are wary of parents getting a false sense of security from them.