I'm quite eager to see production models popping up and some thorough comparisons being done between the 5D III and the Nikon D800.
As it looks from here, the 5D mk3 is a great camera that combines increased full-frame ISO performance with very decent AF and better build quality. As such it's a clear upgrade from the 5D mk2 on the areas where improvement was most needed.
However, it's priced higher than expected (at $3500) and higher than the Nikon D800 ($3000) as well. Now these are two different cameras, but as they're major competitors it'll be worthwhile comparing them.
As it looks now, most likely Canon has the edge on high ISO performance and burst speed, while Nikon has the edge on low ISO performance and dynamic range. AF might be similarly decent in both bodies. But then, Nikon has a special edition (D800E) without an in-body AA filter, uncompressed video shooting, a lot higher resolution (downsampling will again reduce noise)...
If this turns out to be correct, then the Canon is more specialised for action photography whereas the Nikon has much better landscape photography potential, while still being able to do action (in good light). It's doubtful that this difference explains the $500 price difference, though.
Another thing to note is that Nikon does also have a body for professional action photography (the D4), but Canon now has no body for specialized landscape photography.
I think Canon pushed the price too high in the context of the D800. Not that it'll matter for me, I won't upgrade for at least a year or two anyhow. At the same time however, it does become interesting to see some functionality on both the Nikon and Canon sides. The Nikon D4 has backlit buttons and a function to automatically select a minimum shutter speed based on the focal length you're working at, during automatic exposure. Adding auto ISO to that could a lot of magic tricks. I'd love to see such functionality in the more affordable bodies, also the Canon ones. Can't be too difficult, but it'd be so nice to work with!