This is just about the one year anniversary of my switch from Nikon to Canon when I picked up the 5D MkII. I remember when the first rumors about the MkIII came out a year ago, because that was right after I had ordered the MkII. Laughed it off and have been enjoying one of *THE* best cameras on the market for the past year, and it still hasn't been replaced yet. Better yet, I bought it before all the price increases, during Canon's rebate period, and back when you could get 10% off on it through Bing/Adorama, so I could sell it used for a profit now I bet. Awesome camera and it's basically been FREE to me.
Don't really understand why you'd want to wait for a MkIII. Is 21MP not enough? Is ISO 6400 not good enough? The AF system has always worked fine for me, and that's chasing my fast moving kids around too, indoors, in crappy light. If the overall feature set and package just isn't what you want, but Nikon's D700 is, just switch like I did and go for the brand that suits you better. Life is too short to be waiting around for some mythical rumored camera for years on end. If you need something today, buy it today. No matter what there's always something better just around the corner.