It's a question that we here at TreeHugger have been asking recently, and that we have extensively covered. :: Where Did the Bees Go? :: Who is Killing Nature's Precious Bees? :: Colony Collapse Disorder Arrives in the UK :: No Tinfoil Hats for Bees
Straight Dope have an in depth response to the question, as always. They conclude that it's not as serious as a lot of media coverage portrayed. Of course, for the keepers involved it is serious, but from a wider viewpoint the situation is less dire than previously thought.
The problem affects one species of bee, out of 20,000. Despite that species being a very common one, it still only accounts for 30% of pollination. Also, the problem is not as widespread as it seems - some keepers have reported no problems whatsoever. The problem has been seen before, in the 1890s, and it eventually died out, allowing bee numbers to return to normal.
From the article, "The bottom line? No one is certain what's going on, but a lot of the theories can't – by themselves – explain everything we're seeing. More important, the situation hasn't yet risen to the level of a catastrophe (except, sadly, for some of the affected beekeepers). If the same thing keeps happening every winter for another decade or so, then we might really start worrying. But for now, classifying this as a "problem with potentially severe economic impact should it persist" would be a more realistic assessment." :: Straight Dope