For tiny babies in a cradle position it is possible. If their chin is touching their chest they're not going to be getting as much air. You can easily tuck a receiving blanket or burp cloth in behind them to help position them better. There is actually a mama on TBW who is doing some sort of studies on it. I hope it's ok to post this, it's a thread there with more information...
http://www.thebabywearer.com/forum/s...ad.php?t=27841