I've agonised long and hard over the pros and cons of full frame, and in the end I decided that APS-C wins on points.
Noise levels are so low nowadays that this argument for f/f no longer holds. APS-C scores not only on price but also on size, weight, burst rate, telephoto capability and now the live view. I'm really not sure that the perceptible IQ of full frame is really any better than the latest APS-C.
The 5D is a very good camera but also needs very good glass and can still be prone to vignetting. I'm surprised it wasn't updated for PMA, and I'd expect a new version in the autumn.
D300 or 40D? Your call! They are both excellent - and also have a good resale value.
