My expectation is that the announcement will not be a major upgrade, but something to mollify PB15 buyers (those who have white spots, and those who have nothing yet). I've been waiting nearly three weeks for my AlPB 15" 1.25ghz, and I'm told by the dealer ... one of the major mail order houses ... that their backorders on this model go back to early October.
It's interesting that the 1 ghz model is currently available from most sources. This tells me that indeed the screen and latch problems are all resolved. I doubt that the problem is a shortage of 1.25ghz chips, although I suppose that can't be ruled out.
Most likely, it's Apple putting a hold on shipments of the higher-end 15" model so that customers won't get pissed off by receiving what we ordered and having an improved/less expensive model appear a few days later. They've done this sort of thing before, several times. They figure that we'll all forget about the white spots, and even the delay of a few weeks, if we get something that's better than we'd bargained for. And they're probably correct.
It's like being pissed off because your significant other is sneaking around and for weeks, then discovering that you've been thrown a surprise party. All is forgiven, and then some.