Pom, in what important (to you) respects was a 1Ds a downgrade from a 10D?
If I may intrude with my recollection, it was performance in shadow areas, where the 10D was noticeably better.
Judging from what I've read, the 1Ds is not an improvement over the 20D here, but I don't know if the 20D is as good as the 10D.
Jan, I was confronted last year with a choice between a used 1Ds and a new 20D. The 1Ds was much more expensive, but I bought it anyhow. The trade-off for me was very simple: the main advantages of the 20D are lighter weight, faster camera-on, faster image processing. DIGIC 2 may also produce less noise at higher ISOs, but for me that is largely an academic issue. The main attractions of the used 1Ds were simply build quality (when I buy stuff - except printers - I tend to keep it a long time), full frame 24x36 and higher resolution, which I value alot for making enlargements of cropped images.
In my case, the used 1Ds was still noticeably more expensive than a brand new 1D MkII, which would have been too expensive for me anyway. Today, purchasing the used 1Ds would put the total sum of my already acquired 20D body and the 1Ds body at less than purchasing the 1D MkII.
The "resolution" advantage of the 1Ds isn't much compared to the 20D (16% more pixels in either dimension in the final image), but you can get by with lenses that the 20D would strip bare because of its greater angular resolution (37% horizontally or vertically). Cue the arguments con/pro stitching. ::
And wide-angle is of course easier with the 1Ds full frame, which is something that I sometimes miss with the 20D.
As for build quality, the 20D is good, but it's not as bombproof as the 1-series, and of course it shouldn't be exposed to rain or such. I fully expect to be able to keep it at least 5 years, without worrying about it falling apart. A 1-series body should last "forever".
I didn't think I'd appreciate the high-ISO performance of the 20D until I suddenly used it.
All this goes to show simply that what one buys is a very personal decision based on what one considers to be most important for his/her own situation. It's fine for first-time buyers to solicit opinions like this on the web, but as I mentioned above, that is why it is worth putting alot of effort into some basic system decisions relative to ones needs and preferences.
And if my priorities weren't towards getting better lenses, a photo printer, software and other gear, I'd probably have aimed for a 1-series camera myself.