A couple of posts ago, I blogged about several carriers with built-in seats, including the new Beco Butterfly. Unfortunatley, several people have reported a serious safety concern with the Beco Butterfly relating to the tension-release buckles. If somone picks up the carrier the wrong way with the baby in it, the buckles can release, and the straps can suddenly slip. I have seen no reports of injuries, but there have been some rattled moms. This thread on TheBabyWearer has more information. There is no recall, at least not yet.
ETA: The Butterfly is being recalled.