This may be obvious, but I'd never thought of it before. Recently I had my G5 in Target Disk Mode, hooked up to my PowerBook, and I wanted to sync my iPod (3rd generation) with the PowerBook. However, I was flummoxed by the lack of an available FireWire port, since the only one was in use. Since FireWire devices can be daisy-chained, though, I decided to try hooking the iPod up to the other FireWire port on the G5, and lo and behold, it mounted just fine on the PowerBook.
I haven't actually tried this with any other devices, but I assume it would work just as well with external drives, cameras, etc.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007071220555530