I came across this theory awhile ago and it makes a lot of sense to me .... that the flame-retardants and chemicals that baby mattresses are filled/sprayed with could cause SIDS. You would not believe what is in mattresses. Its sickening and very, very disturbing (and should be a crime!). It would also make sense why babies on their tummies are more likely to die of SIDS - because they are closer to the mattress and breathing in the fumes more closely. Used mattresses are the most toxic.
There is a mattress cover called Bebe Safe that is made from a safe, non-leaching plastic called polythylene. I have these on Jayna and Abrams mattresses and I just found one made from the same safe plastic online for my bed (for way cheap, yay!). I buy the Bebe Safe covers at Eve's Best. Click there to read articles with more information. I figure for $35-40 its worth even the possibility of saving a life.
These covers have been used for years in New Zealend and ZERO babies have died of SIDS whlie sleeping on a mattress properly wrapped with a Bebe Safe cover. Not one. That says something! (must also avoid anything besides cotton. even the "organic" mattress pads sold at places like BRU are a total marketing scam - they have polyester or plastic/toxic backing with an organic top!)
HERE is a discussion about using them on mothering.com
If you have a baby sleeping on a crib mattress i highly suggest you look into it.