I think the fact that I am seriosuly considering three cameras that cost $600, $830 and $1360 would lead most people to question my sanity, judgement, and whether or not I am in a judicially ordered drug testing program. It's how I feel, after all!
I was wondering that myself. However, I think it is better to have "too much" camera than to end up stuck with a camera that is missing a key feature or two. That's usually a purchase you eventually regret.
The 20D looks like a great camera. I've seen the DCRP review and the 20D takes excellent pictures. What lenses would you get for it and how much more will that bring the total price? I believe there is a 17-85mm image-stabilised lens that you could use with it.
The Pentax *ist DS also looks like a fine camera. I'm attracted by its small size and weight and it has a DOF preview too, which I believe is something you are looking for.