I got an idea... not sure it will work. If I made txt-files run through PHP with an AddType in htaccess i could have a script in my robots.txt that recorded the IP. Then I would KNOW it was a robot without having to mess with robot-IP-lists.
Also some browsers and internet accelerators and such look at robots.txt. And, robots don't always access robots.txt prior to accessing other files. Some robots only access robots.txt once a month or so, and continue to request files all month long.