One Cause of SIDS

Scientists and researchers have attempted to explain SIDS in a variety of ways. Genetically some see an association with a couple of genes that control the heart. The lack of serotonin receptors in the infant brain have also been been blamed, because they are responsible for telling the brain how much CO2 is in the blood. Just as with the flip-flopping over whether eggs are good for us, officials with the American Academy of Pediatrics Task Force on Infant Sleep Position decreed in 1992 that infants should no longer be put on their stomachs and instead be put to bed on their sides or backs. This may have resulted in lowered SIDS mortality. The numbers can be deceiving however.
See SIDS deaths have declined, while the overall number of infant deaths has not changed. What appears to have happened, though its impossible to know with this little information, is doctors have begun diagnosing deaths that formerly would have been SIDS as something else.
Other explanations for SIDS have been correlations found in the research. SIDS tends to strike in the winter, when babies are more covered and ventilation is less than what it would be in the summer. Babies in high wall cribs are more susceptible. Babies with SIDS don't smother, because there is no external force keeping them from breathing. Babies with SIDS do asphyxiate, that is they don't breathe deeply or enough to maintain the oxygen levels in their blood, and their bodies apparently don't tell them to wake up.
The "toxic gas" theory that explains SIDS as the result of toxic gas created by a fungus that lives in bedding washed with phosphate containing detergents has been found to have some serious flaws. It's still a contender though.
SIDS is caused by a variety of factors we'll no doubt find out someday soon, not by any one culprit. Even when we know what those culprits are, we'll still be limited to protecting our infants by doing mostly what we know now. It's unlikely major changes will occur in this list.
Make sure baby sleeps on back or side with plenty of support.
Babies need lots of fresh, and heavily oxygenated, air. Make sure you've got circulation, especially around baby's face.
Make sure baby's sleeping surface is firm and won't cave in like our damned air mattress does.
Don't do things that are going to lower the oxygen level in your air like smoking, burning anything, using air sprays and those silly plug-it-in Glade thingies.
For a longer, more complete, list go look at the Centers for Disease Control webpage.
That's it from me.