I just had my baby care class tonight and the pediatrician who taught a big block of the class kinda chuckled when we asked about bumpers. He said their only purpose is decor, and that they are really quite silly. He didn't say don't use them, but said once baby starts to move around at all they really recommend not having anything in the crib.
He said that one of the theories about SIDS is that the baby isn't taking deep breaths so the air around baby gets sort of caught up in the immediate space around him/her (and that air isn't good oxygen since it's been expelled already by baby), and it's possible bumpers may not help that---in other words, good air circulation is helpful and bumpers obviously don't promote it.