Yes, the lsof [list open file descriptors] does show what processes are accessing the device. One issue I ran into is ---- it is impossible to umount a USB device if you open a terminal session and traverse to the mount point or one of the subdirectories below it. This is definitely logical and the prudent thing to do is to shut down the terminal session. That did work for me and I was able to unmount.
My distribution is Ubuntu 8.04 [Hardy Heron].