As you know the world of photography is changing rapidly with the addition of full frame cameras at (what once was) crop sensor prices. Specifically, the Canon 6D was recently announced, along with the Nikon D600 just a few weeks before it. I primarily shoot Canon, so I cannot comment on the Nikon D600 really, but I did want to give you my opinion on how both of these cameras are changing the field.
Since I shoot stock, the burst frame rates, the better AF and the increased Dynamic Range of the 5D Mark iii would just be overkill for my craft (at it's price, anyways). A 6D though, with built in wifi to get my images to my computer without an adapter, and better ISO performance, and (possibly) better IQ, may be just what the doctor ordered for me personally.
We'll have to wait for real world reviews of this new wonder camera to see if it is an improvement over the massive game-changer that was the 5D Mark ii. Until then, we can only speculate (and speculate we shall!). But if it is an improvement, I could see the benefit of upgrading.
Just my 2 cents.