I was probably in the same situation as the OP. Weekend hobbyist, with mostly FF needs for portrait, landscape, and travel photography, and hence the purchase last year of the 5D. But I also do the occasional action/sports photography, mostly motorsports, kid's sports, and dog action.
I was drooling at the prospect of getting the Mk III (or the Mk IIN before the stores sold out), for the burst rate, AI Servo tracking, 45 point focus points (reminds me of my EOS 3), and other features that lend itself as the sports/action camera of choice. However, I could never let my 5D (I am a FF addict) go for a crop camera, and I also wanted to budget for some more lenses before I was willing to spend big on a 1D body. That caused me to wait on the Mk III, that is, until the 40D came out.
I now have the 40D, and it is a very capable camera. I've used it to do some of my action/sports photography, and found it adequate and capable to meet my needs. I haven't been spoiled yet by a 1D, which may be one reason why I'm currently satisfied, but the bottom-line is that I've now got the speed to capture the images that I want with the 40D. But, ask me 2 years from now, and my desire for the Mk III may be back.
As another poster indicated, if you're not a shooting pro, then I think it's a matter of whether you have the means and the inclination to invest in the Mk III. For the money, I think the 40D is an excellent sports/action camera.



