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.