And here is why:
http://www.macdailynews.com/comments.php?id=P1256_0_1_0
Some people reported seeing the price drop already, maybe at the same time as the the famous G5 Desktop picture.
I think they will continue to sell for a while yet, and we will have to wait a month or two for newer powerbooks to arrive.
This is only my opinion, and I will probably be completely wrong.