I recently bought a Canon 7D (see other post) and today I spurred for the 5D mkII. Full-frame is so much sweeter and wide, and I can put the 24-70mm f/2.8L USM to proper work. It's too narrow on cropped on the 7D because the APS-C CMOS sensor.
Here's my advice: If you shoot action, get the 7D. It's insanely great value.
However, if you shoot portraits, low light, or mostly landscape, GET THE 5Dmkii! It is AMAZING! :)