This is the problem Im having as well, I've been trying to decide to pull the trigger and get the 5DMKII now but I feel like as soon as I buy it canon will "announce" the MKIII. I know the camera is "dated" as someone posted earlier and I know it doesn't exclude the fact that it's still an amazing camera. Not sure if I should buy now or keep saving.
Prices are low on the 5D Mk II, probably because of speculation of an upcoming 5D Mark III.
Everyone gets excited about technology (and those who read rumors sites obviously have some interest in new developments and tech), but for all the buzz surrounding new releases, gains in tech are incremental.
Take ISO (for example since it was mentioned here). On the ISO component of the DxO Mark test, the Canon 5D classic (not the Mark II) is the 9th best of the cameras in that database. It's within half a stop of the 5D Mark II, and within 1.25 stops of the top ranked camera, the nikon D3S. The 8 cameras in front of it are all full frame -- no APS-C.
Even the old 5D classic not only is still an excellent performer, it has the full frame advantages -- as neuro points out, it's like an instant upgrade for all your lenses. After all, why drop a bundle on f/1.4 primes and f/2.8 zooms and then mount them on a body that makes them f/2.2 and f/4.5 ?
So the calculation at least for me was, from the crop to full frame was a substantial upgrade, but the reality is that from one version of a camera to another, the newer one will have some nice new features but the bump in performance will be incremental (even if the reviewers rave as if it were revolutionary, a closer examination will typically reveal otherwise)