The only problem I had like this, was like davidgro said, with a 4 GB A-data sd card sucking too much power.

There is a cmd line tool, battery-status you can install that will log battery level at regular intervals. It might help you determine if the battery drains at a regular rate or all of a sudden.

I use advanced backlight on my n800 all the time, and I have had no battery problems with it. You don't have it set to allow zero backlight?