MSNBC reports "Bunker Buster" bombs will be used
These bombs are designed to penetrate deep into the ground before detonating. The resulting shock wave can collapse the caves where bin Laden and the Taliban are believed to be hiding. According to this article, tests have shown the bomb can penetrate over 20 feet of concrete or 100 feet of earth before detonating.
MSNBC did make a mistake in their article. These bombs don't cost $18 million each. According to the Federation of American Scientists, the FY97 budget included $18.4 million to produce 161 bombs. That's still over $114,000 per bomb but a lot cheaper than $18 million each.