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In the linked article, the Blizzard CEO states that WotLK will launch sometime between October-December 2008. I have thought this all along as well.
My thought has always been mid to late October. The reason I say this is because of all of the hype around beta codes at WWI and other events. We have heard nothing about beta/open beta codes at Blizzcon. I think Blizzard is anticipating the launch to be around this time and plans to either celebrate the launch (maybe the week of) or really build up for the launch a week or two later.
It is just a guess but I strongly feel the expansion will be out before December.
It seems that September-October will be a huge time for MMO's: Warhammer Online, Mines of Moria and WotLK just to name those off the top of my head.