Yeah, I was a bit uncertain about the new name, too. The name "PowerBook" has been a trademark for so long that it has practically entered our lexicon. Everyone has heard of PowerBook. You'll hear the word used on The Simpsons or Frasier.
On the other hand, the new dual-core processor deserves a new name, and MacBook is going to be a very simple and straight-forward mnemonic, so it should catch on.