Depends on sales of the PHB2. Purely on antecdotal evidence, I would guess it's been a very big hit, perhaps the biggest since the Eberron CS. If that's the case, I would expect PHB3 to get approved and the developers to start accumulating ideas for it, probably to release late next year.
On the flip side, if the PHB2 doesn't provide a spike in sales, I think WotC will (rightly) see it as a sign the market for 3.5 crunch is starting to fade. It's an exceedingly well done, mass-appeal, player-focused book with an attention-grabbing title that actually manages to fill holes that haven't been filled either in-house or by major 3rd party publishers. If that doesn't grab people, nothing will, and it increases the likelihood that 4e will move up in the development cycle.