If it wasn't successful, you could use the firmware restore CD that came with the box, or create the CD to do that, but again, that only works for failed upgrade attempts.

I've had to do this once in the past, and the easiest way I could find was to just take it to the local Apple Support shop and get them to do it. They did it for me while I waited, and it didn't cost me anything but time. YMMV.

But good point... I'm not in the habit of backing up firmware.