fstat | grep "Outlaw"
fstat is a file status program which identifies open files, and then the pipe (vertical bar) passes the output of fstat to grep, the UNIX search program. I asked grep to find the word "Outlaw", which was part of the disk image name. The output of the command was:
robg TruBlueEnv 804 11 19 drwxrwxrwx 264 r /Volumes/Outlaw-0.1b1
TruBlueEnv is Classic, which somehow had the image file open -- even though I had no Classic apps running! At that point, I returned to the GUI, opened System Prefs and stopped Classic. I could then eject the disk as usual from the contextual menu.
So if you've got a stuck image file, take a look at fstat with a search on part of the volume name (to shorten the returned list!) to find what's making your volume busy. NOTE: There may be easier ways to do this (UNIX wizards, any thoughts?) but this was the first one that came to mind when I was faced with the problem.

