This seems to work
http://www.appbrain.com/app/com.keyes.screebl.lite I'm not sure how this would affect the battery, since it polls on the phones orientation sensor.
BUT..... It's ridicules that there's no built in function that checks to see if it's plugged in and keep the back light on.
If someone finds an app that works on weather the phone is plugged in or not... please post here.