At the ParentsPlace "Ask the Doctor" web site, the answering physician
strongly counsels against co-sleeping based on his personal experience
with two families where the cause of infant death, in his opinion, was
truly overlaying.

http://www.parentsplace.com/readroom/dr_answers.html#sleep

You can read his advice as well as followup comments from other parents.
 I think it is important to address this issue from a risk-benefit
perspective.  There may indeed be isolated cases of death from
overlaying even in non-compromised (drug, etc.) situations.  On the
other hand, there are indeed isolated cases of death from wearing seat
belts while riding in a car (can't get out in fire or flood for
instance).  Do we stop wearing seat belts?  No, because in general seat
belts save more lives than they claim.  We need to promote shared
sleeping as a medically beneficial, even life-saving, activity, not
just a warm-fuzzy thing to do when we are too tired to get out of bed to
feed our babies.

~~ Nancy Penney