Ask yourself...
1) Do you have US$1.5k or US$3.3k or US$8k to spend?!?! Very different strokes for very different budgets.
That's $4K - the 1D IIN, not the 1DS II As he's actively considering all 3 of the named cameras, this is presumably moot. He'll get whatever will give him the best life expectancy.
2) How important is wide angle, how important are long lenses, and what existing glass do you have to meet those needs? or what new glass are you considering, and in place of what? (It helps decide 1.6 crop vs. FF needs)
3) Do you ***really*** think you will exploit the difference in pixel count?
Both the 1D IIN and the 20D have 8.2 MP; the 5D only has 50% more, a minor difference once you get up to this large a sensor (resolution increase on the order of 25%).
The real deciding factors are the crop factor relative to OP's current lenses and other features such as frame rate, buffer size, max. AF aperture, interchangable focussing screens and levels of customization possible. The show-stoppers, for me, would be crop factor (5D or 1D IIN) and buffer size (3fps is fast enough, but a 5-frame RAW buffer's too small)(5D or 1D IIN). AF at f/8 would be nice (1D IIN) as would the interchangeable screens (1D IIN or 5D). Not having used a 1-series, I don't know how much of the personalization I'd want, but the ability to specify the folder names would be convenient (1D IIN or 5D). I suppose I could get used to having mirroring of my photo captures. Considering the features, I think the best long-term buy would probably be the 1D IIN, although cost considerations might make the 5D more attractive. If it's among those 3 cameras, the only ways the 20D can compete are if you only shoot long lenses or if up-front cost is a major factor.
