Anyone seriously interested in a portable power plant for their digital equipment can check out http://www.digitalcamerabattery.com The 30 watt kit (their smallest) starts at $426 USD. This kit comes with a black 30 watt Digital Camera Battery, standard charger, carrying case with belt clip and D-rings, power cable to fit any D1 series camera and a high voltage cable for the SB28-DX flash. The units can be purchased directly from the manufacturer, and they also sell thru several dealers in the US, and a few around the world.
I haven't tried this out personally, but did read some good reviews several months back. Impressed me enough to convince my boss to buy a couple for the shop. The pity is, they won't get here in time for me to play with before I move. So I won't even get a chance to play with them and see if all that I heard was true.