Just my opinion, but all this banter about which camera is better, is the price point justified, et cetera, is all baseless until we see side by side comparisons of the photographs. After all, the camera is a means to an end, right? The 5D Mark III has all the right ingredients to make me think it's going to be a beast at producing results. The autofocus helps you get a perfectly focused print, the 100% coverage helps you properly compose, and the three years of technological advances applied to roughly the same megapixels should really enhance the quality of the image output.
The whole "jumping ship" thing gets thrown around every time a new camera comes out, including Rebels. Seriously. Will there be some people who will sell all their Canon gear and buy Nikon stuff, requiring them to relearn controls and muscle memory and basically flush thousands of dollars? Sure, but they'll be few and far between and dopes. Will there be Nikon users who jump ship if the Mark III outperforms the D800 by two stops in low light image quality (for most photographers, a COLOSSAL advantage)? Sure, but they'll be few and far between and also dopes.
Look, if the 5D Mark III is too much for what you're getting in your opinion (i.e. the enhancements don't really do a lot for your finished product), take a look at the Mark II. The results from its sensor are absolutely awesome and at $2,100, it's a steal.