The 5D Mark II is a great camera. I know several people that have and love them, but I can't bring myself to buy three+ year old technology, believing that a replacement is right around the corner. I have to admit the wait is killing me.
Based on Moore's Law, Canon should be able to get four times as many pixels in their sensor, now, as they could have, three years ago. Of course, it's not always about more pixels, as has been pointed out. There are trade-offs, such as reduced heat, power, noise, etc. Still, I would be surprised if Canon couldn't make a FF sensor that is one or two stops better and still increase the pixel count.
The 5D was announced on August 22nd, 2005.
The 5D Mark II was announced on September 17th, 2008.
Here are a few things that changed, during those three years (8/2005 to 9/2008):
Megapixels 12.8 increased to 21.1 (60% increase)
Maximum ISO 3200 increased to 25,600 (3 Stop Improvement)
Added 1080p video
Given a similar 3 year time-frame (8/2008 to 10/2011), I believe it's possible that Canon could produce a 5D Mark III with:
Maximum ISO 204,800
Canon has a lot of options and trade-off's to consider when implementing newer technologies and I don't necessarily expect to see a 33.9 megapixel camera with 204,800 ISO, but in theory, based on the leap from the 5D to the 5D MKII, they could.