Blizzard would certainly love to have it ready by mid-November, as that coincides very nicely with the start of the Christmas shopping season. But they do seem to have a commitment to quality, and when your game has ~10 million subscribers (and possibly more by then) you do not want to have a disastrous expansion release. Everquest had some horrible expansion releases, some that took weeks before the major problems were cleared up. Blizzard execs would be lynched if that happened with WoW.

I would guess early '09, just to allow for the likelihood that they won't rush it out the door until they're sure it's ready, or as ready as it can be.