I think the smoke kinda "scorches" the tastebuds, which makes them less sensitive, probably in pure self-preservation. Also, that junk that gets deposited in your lungs as tar? It's all over your tongue, too, in the little furrows between tastebuds. After you stop smoking, it takes some extremely variable amount of time for them to get over the "shock" of smoke on a constant basis, and for all that gunk to be cleaned away.
Andria
EDIT: one other thing -- most don't know or realize it, but there are a considerable number of "tastebuds" in your palate, the roof of your mouth -- they're just as affected by smoke as the ones on your tongue.