Apparently during the writing of the new update to the firmware the iPod requires more power than USB can offer. My guess is some computers could power it fine over USB, but the USB standard is lax about power requirements, and some computers offer less power than others across their USB bus.
While the instructions after the firmware updater are run, and the graphic that appears on the iPod imply you MUST use the wall power adapter, I’ve found that connecting the iPod to my car cigarette lighter adapter, and to my computers 6 pin FireWire port also allowed it to flash its firmware successfully. So apparently it is only 4pin FireWire and USB connections that require an alternate power source.
This was a problem for me, however, as I normally carry my iPod around without the power adapter, or FireWire cable when I’m traveling, as my home laptop is a PowerBook, but my work/travel laptop is a Windows machine that I connect to via USB. Luckily I was able to connect it to someone’s car charger while I was traveling to complete the update, however, I think Apple should put a large orange sticker on the USB cable included with the iPod warning users that you could be stuck with a non-working iPod if you travel with only the USB cable and NOT the power adapter, and choose to do a software update. They should at least, when running the updater software, warn the user they will need a power adapter to complete the firmware update. They allow you to run the update enough off of USB to disable your iPod, but not restore it.
Anyway, here is the graphic on the iPod screen you will be presented with when you do a firmware update, and do not have a power adapter:
The safest thing to do, I guess, is never do a firmware update when traveling without your power adapter.
I found some discussions about this here as well: iPodlounge