For all you people saying 3rd quarter 2010: The final patch of this expansion is already on the PTR. Say it comes out in a month or two, that would leave almost an entire year with no patches or new content. I would see a lot of people quitting during that time.
Blizzard wants to keep the content rolling because people tear through it so quickly. They don't want you getting bored with nothing new to do for that long. I would guess this new expansion will arrive when the robin makes his nest. Springtime!