For the moment I've only been using the line-out stereo jack on the dock and relying on the battery. I've been meaning to rig up the iPod firewire cable for power so I can charge it in the car, too, but that requires me buying some stuff (a second iPod firewire cable and the cigarette lighter adapter thing). I've been sort on $$ so I haven't done it yet, but I hope to get to it soon.
I figured I can connect the standard firewire end of the cable to the cigarette lighter adapter and then get a spare cigarette lighter (from Pep Boys or whatever) and wire that in to the electric and store it all behind the dash out of sight. That is the plan for now. Comments or improvements welcome.