Did you try removing or replacing the SIM card with a different one? I remember reading some reports when N900 first came out about people with a flaky SIM card that caused battery drain. It would be interesting to try. Also try removing your SD card if you have one inserted. How it would be related to charging, I have no idea... grasping at straws.

Also try Offline mode, if you haven't already. It should normally last several days idling in Offline mode (but of course it's no fun...)