I was going to update my previous post to reflect Alex's comments and refine my guess to 8 weeks (2 weeks to patch + 6 weeks to Wrath).
Turns out that would have been pretty close. November 13th 2008 is Northrend Day. Seems to be all over the blogosphere already!
Oh and the patch is about 950MB - I recommend setting the options to not throttle background download and leave the PC on overnight!