Apologies for cluttering the thread, but I thought a summary of the options might help.

There are essentially three categories:

1. For about $30 retail, you get a cord that is Empower-only. This will power your Mac so you can use it without draining the battery, but will not charge our battery and could even damage your battery if you don't remove it. They're cheap and very light, though. Available from kensington and madsonline, and maybe others.
2. For about $80 retail, you can get a Car/Air adapter that will plug into both Empower and cigarette lighters (like in cars and on AA). These include a "brick" that converts the electricity so it's less likely to harm your battery, and may even charge it. Available from iGo, Lind, kensington, targus, and probably others.
3. For about $120 retail, you can get an Air/Car/AC adapter that does everything the $80 adapters does, plus replaces your regular AC adapter so you only have to carry one power adapter on your trips. Available from targus and kensington at least. iGo and others probably have them, too, but I haven't checked.