The term "immovable joint" seems like a bit of misnomer to me. They seem to all allow SOME movement under the right circumstances (sort of like how we think of plants as being still, but they can move more than you might think). I guess there's sort of a continuum in how much joints can move--immovable joints, gliding joints, etc.
@rugbygirl - You're right about the pelvis. OBs talk about the baby's head not fitting through the pelvis sometimes, but changing positions can change the shape of the pelvis quite a lot!