Whilst still calcified, teeth are not bone in the sense that his skeleton is. They grow and change in a different way. It's also possible when his healing factor adapted to the infusion of adamantium it forced the metal out of his teeth.
Furthermore, as HNL points out, when the adamantium was first injected, it was only into his bones, not into his teeth. I doubt it would have gotten very far before solidifying however, so there must have been some factor spreading it about his body. Whatever this was probably missed or couldn't effect his teeth.
The adamantium infusion (especially as shown in the Origin movies) appears to involve drilling into all the major bones and feeding the alloy into the marrow. I doubt his teeth were ever drilled into. He has to rely on his usual healing factor to survive blows to his teeth.