The extra 1mp resolution is not going to be visible, (its a 22mp camera).
Improvements to focusing etc are welcome and it will no doubt be a slightly better camera than the Mark II but it wont be significantly better. If you have lots of Canon lenses then its an option, if you can start from scratch then Nikon are where things are at these days.
Update: Canon are pricing the camera at £3,000 in the UK some £600 more expensive than the new Nikon D800 with its leading edge 36mp sensor. Even without the sales tax the camera is still far more expensive than in the US market.
Canon have also release a wifi transmitter for the camera at £790, yes, £790 for a box of electronics that cost at most $40 to produce, clearly Canon are not planning on selling too much kit in the UK from now on.