Having had a look and asked some questions on the Focus on Imaging Canon Stand at the NEC Birmingham yesterday, these are my first initial thougths.
The cameras main attribute 22mill pixels main feature, if using video option exactly the same as the MK2 slow focus and a bit noisy, so the mk3 is no better in this area, I asked about the enviromental safeguards has it replicated the Eos 1D/s no was the reply it has the equivilent of the Eos 7D so rain water, dust etc can invade the unit, it has more info available to the user and a quieter shutter mechanism, more focusing points, and as far as I could see without an indepth review thats about it.
Would I buy one, I have had my 5D Mk1 for 5 years and my mk2 for 2 years, used the video option on the Mk2 twice, did not like it, its a still camera, if I wanted video I would use my panasonic HD or my trusty Canon X2. The update from my Mk1 to my Mk2 was a leap, this in my opinion! at the moment ( and it can be changed ) is more of a hop over to another step on the same staircase, would I pay £3000 to say I own a Mk3? NO not at all, not when I could buy two Mk2s for the same price, the technology has not gone ahead that far to justify changing to Mk3.
In short Canon I feel, have failed here with a very costly upgrade rather than a remake of a fantastic marque, sorry to say it but on first contact it falls short of what I would be looking for if I were to upgrade my kit.