What I did is set my computer to hibernate after 1 minute of inactivity while running on battery. I then bought a power adapter that plugs into the cigarrette adapter in the back, like this one: http://www.compusa.com/products/prod...13121&pfp=cat3
This way as soon as I shut off the engine, the power from the accessory jack gets shut off. The computer switches to battery power, then gives it another 1 minutes running off the battery before it hibernates automatically. Since I drive my car every day, I never have to worry about the battery dying while its in hibernate mode, because it charges as soon as I turn the key.