This incredibly compact DC step-up converter is what I used. It fits neatly inside the 3G case behind the hard drive.
http://www.pololu.com/product/2117
Here are the traces for USB and Firewire power.
[image|459227]
Problem is, the iPod freezes when plugged in. It seems that the iPod refuses to function when both USB 5V and Firewire 12V power are being supplied. And you can't cut off USB power to the device, because that's what the iPod uses to power the chips that interface with the computer. How did you get around it? Or is the iPod non-operable when plugged in via USB?