I have the biggest focus problems with simple, straightforward shots. Airplanes haven't been much of a problem (at least, nothing I can blame on the camera), but put a group of objects on a table and I can't seem to get a sharp shot on them. Happens with both my Canon and Sigma lenses. I contacted the commercial photog I use for work--he's also got a 10D--and he's experienced the same problem with his, so I know it isn't just me.
After prowling the 'net and some of Canon's forums (I forgot where I found them now), I learned that a lot of users have experienced this, and it seems there are firmware upgrades available on Canon's website that may or may not address the issue. I haven't tried one yet because I did a focus test in the meantime and the camera didn't do badly. Here's the one I tried:
Considering that I'm still fairly new to digital photography, I decided it made sense to not get too hung up on this until I got more experience with this camera. It does seem as though things are improving for me, but your mileage may vary--please post back if you learn anything new.