Meg, I have no idea why SIDS would be unheard of with co-sleeping (and I doubt it could prevent the SIDS cases that occur during day sleeps) but in my case I woke up in a flat panic one night because Imran had stopped breathing. It was only after I gave him a full-on shake that he took a huge gasp for breath (but didn't wake up). Had I been sleeping in a seperate room and relying on a monitor I never would have woken up and my gorgeous little Imran would be another SIDS statistic.
It took me months before I could sleep well again after that.