Question: In "X-Men Origins Wolverine", Logan undergoes an operation to have Adamantium bonded to his skeleton. 

Why doesn't the Adamantium bond to his teeth?

Answer: 
In humans, teeth make up part of the skeletel system but they are not classified as bones. Teeth are made up of Dentin and Enamel whereas bones are made up of Osseous tissue. 

So I imagine the adamantium only bonded to the Osseous and not the Dentin or Enamel which results in adamantium bones and shiny white teeth.
I think the most simpliest answer is that the adamantium was **injected into his body**, where it covered his bones. In order for his teeth to be covered the injected metal would have had to ooze out of his gums into his mouth.