Same problem here. Even when the N800 is off, the battery goes of.
This happens after upgrading to OS2008.

I noticed that this is tha case if I do the following:

1. If I turn of the N800 when it is not pluged in, no problem.

2. If I turn off the N800 when it is pluged in, and then plug it off, the battery goes off over night. I have the feeling that this is the problem.