The exact word that G5 Powerbooks would be announced October or November was given by someone at Apple. Normally I would have passed such a thing off, but when it comes after informing us of a hefty supply problem for iBooks and Powerbooks, it makes it considerably more legitimate. To reiterate, we have been told that orders taken before the end of the 17th of September will be honoured by the end of the month. All orders taken afterward will not be honoured until October or November. A potential 2 month delay on laptop orders is a pretty big deal. It sounds awfully similar to the iMac situation, doesn't it? Imagine if Apple hadn't announced that there would be a new iMac at the end of August, and had just kept taking orders. There were no machines, so all the orders would have been delayed, and then honoured with new machines once the new one was announced.
As for supply problems with the G5 chip itself, IBM has more or less cleared the hurdles, and is producing them in fine supply. Apple held off until now to release the iMac because they knew supply would not be sufficient until now. Considering Powerbooks will likely use the same chip as the iMacs, it shouldn't be all that much work to provide enough chips.
Certainly, I'm not holding my breath on this all, and I'll admit to being somewhat skeptical. However, the precise words were "G5 Powerbook", and it's associated with a supply deficiency that explains it perfectly.
