There has been research done by a Dr James McKenna (director of the Center for Behavioural Studies of Mother-Infant Sleep at Notre Dame University). Studies he has done suggest that the mother's presence has a direct effect on her infant's physiology in the following ways that help protect against SIDS:
The arousals that some of you mention are meant to be important as it stops the baby from sleeping too deeply.
Mother and baby have harmonious sleep ie they are in the same stages of sleep (REM or non-REM).
Babies that are prone to irregular breathing and apnea experience these episodes during deep sleep.
Less deep sleep means less risk.
It is also thought that the baby hearing you breath reminds them to breath.
I also believe that if your baby is close by you instinctively know if something is wrong. You wake and sort it out.
I would be interested to know if there are any statistics on the number of cot deaths that occur when the baby is in their own room compared to babies sharing their parents room.
On a personal note - I did co-sleep and dd has just moved into her own room (she is almost 9 months). It has only been 3 nights but last night she went to sleep (in her cot) at 8.30 pm she woke at 11pm settled back without a feed and slept from 11.30pm till 7.30 am. No tears. It has been, so far (touch loads of wood), much easier than I expected. But I do feel she wasn't ready to be on her own any sooner. It also gave me chance to prepare us gently for the move - I have been getting her to sleep without always feeding her and I had the cot with the side off next to the bed so I could roll her in so she wasn't sleeping in my arms. This has all taken time and I gather from a lot of postings some of you just don't have this patience. But I truely believe you have to respect your baby as they are undergoing so many changes and they need you to guide and support them. If your baby is a good sleeper it makes thing so much easier but not all babies are and to "condition" them to be felt very wrong to me.