Last October, the rumor mill was certain that a 5DIII was coming in early 2011. I bought a 5DII, and have shot thousands of images with it so far. I know, for a fact, that those images with the 5DII are much better than images I cannot take with a nonexistent 5DIII.
Seriously, if you need/want it now, buy it now. If you are happy with the gear you have, wait. The 5DIII will come out someday. You can always sell your 5DII and buy a 5DIII, if it turns out that the 5DIII is such an improvement.
EDIT: deleted my edited quote, from Isaac who likes to cross post and who's brain we aren't following, I think, but maybe not, since whoever posted it deleted their post, leaving my edit as a non-sequitur.
With the current price gap, the next 5d might fill up the hole and get a 4000+ tag. With that happen i dont think the mark ii price will go down. It might will retain the current price. Are you going to buy a 4000 body? If not might as buy the mark ii now. You will never know what is going to happen next. As dilbert said, the 1D X does changed a lot of things.
I was in the same position as Neuro last September...
I ended up buying it and have taken some beautiful shots... It really is a good camera...
You can pick them up quite cheaply now and if you have a great 6 months taking photos with it and sell it when the mk3 comes out and lost 25% . Then you haven't lost much but gained some awesome pics
That was my thought process and it turned out to be a good one as it's 13 months later and we are no closer to a release date !