I use my 10D since a few months and I simply love this camera, great quality, great handling... I don't do sports though. I can imagine it's a good tool for sports (if it's fast enough with 3fps like somebody mentioned before). The multiplication factor makes the focal lenght of every lens longer by the factor of 1.6 - good for sports
? this camera produces zero noise at ISO 100&200
? ISO 1600 is still very usable - good for sports
? colors are very natural - also important
? AF is quick and right on (some people have had backfocus problems though)
Since you want to use it professional, I guess I don't need to tell you 'bout lenses...
batteries: I get approx. 300-400 shots with one battery. A 256 MB CF card gives you about 100 pictures in high quality jpg mode. So just get nuff memory (tip: stay away from Microdrives). I don't bother shooting RAW (unless it's some high tech studio stuff) coz the postprocessing is just too much work... I believe for your task it doesn't matter anyway.
Concerning prints in offset print quality (300 dpi) I'd go up to DIN A4, you can sqeeze more out if you really need to
hope this helps (oh, you can read some more here
http://www.dpreview.com
one more thing: compared to film it acts more like shooting slide film (highlights tend to blow out) it helps a lot to expose for the shadows...
Jeff: Canon's 1D was optimized for speed, 1 Ds for quality 8)