The 5D is a milestone camera, like the 1Ds was. I'd have no hesitation in recommending it to anyone. The upgrade will be better, but it's not at all clear if it will be better in the sense that the 40D is better than the 30D, ie. no significant improvement in basic image quality but a lot of useful new features such as faster frame rate and LiveView.
An upgrade from 12.7mp to 16mp is hardly more significant than the upgrade from 8mp to 10mp of the 40D.
My goal is set on a 24mp sensor. If Sony comes out with this before the Canon 5D upgrade, I might well switch to the Minolta-fit Sony. I have a number of Minolta-type lenses, including a Sigma 400/5.6 prime, a Tamron 90-SP/2.8, a Minolta 35-105 zoom and a Minolta 50mm prime. I'd like to use them. I hate waste .
Oh! I also forgot to mention the Sigma 24/2.8 prime (Minolta fit) which is considered to be better than the Canon 24/2.8 prime.