I would think it is almost a given or necessary that Canon takes the 4K step with the 5D. There are some cameras in the market right now that shoot 4K and in the $2500-3000 price range. My guess is that they would support external recording with the Atomos. 4K gobbles up storage like nobody's business.
For those of you not into video, shooting video with a 5DIII gives you many alternatives using all the Canon lens, and it is much better in low light situations. We also use camcorders, which are easier and quicker to use. It depends on what your needs and goals are that determine which to use for a given situation.
I shoot both at the same time, 80% of my time and it is really convenient just to dial the shutter speed and ISO up and 1DX becomes one of the best video tool in the room. I have only one body and two lenses with me.
If 4K and the 70D autofocus is introduced to 5D, I would sell my 5D MK III, even at half price, to fund it.
Raw photographers outhere would appreciate the advantage of not shooting in JPG anylonger.
With the arrival of Davinci 11, color grading will finally be a fun.
I will be more exited in getting that than buying my dream lens 200mm f2.0 (I don't have the money anyway).