Personally, I will stay with Canon. If the MKIII doesn't meet expectations and comes up short against Nikon's offerings, it just means Nikon has won the battle, not the war. It's always back and forth and it's an expensive past time selling all your depreciated glass and equiptment to switch when in 2-3 years, you may be on your way back.
I think it's sad how people think equiptment make them a good photographer. It helps, in some cases, from a technical standpoint ie. more megapixels but I would say the vast majority of people on this forum are not professional photographers, who think that switching companies and getting the newest tech or upgrading with the current company they shoot with will make them vastly better photographers. I love tech just as much as the next person, but work more on your craft and become a better photographer and what you can do with your skills and talent not what the camera can do for you.
So all of you are switching...buh bye, have fun... In a couple of years, you'll be possibly switching again...