Ive been experiencing too a drain of the battery in my N800 running Diablo. But not before a couple of days ago. I have my N at my office from 10am to 8pm with the charger connected (and obviously charging). However, I was on my way back home last night and watching some vids downloaded with myTube (its a 45 minutes ride) and then stashed my N in my backpack until this morning when I tried to get my emails when I saw that the battery was all gone. (Ive watched vids before and even movies on mplayer and never had that battery problem)
Batterry-satus wont work in my N (incompatible).
One simple idea that crossed my mind was to uninstall fm radio, since I thought that even though no earphones are attached to the device the fm function in the N keeps working (correct me if I'm wrong) in case of an accidentally activation of the play button in the fm radio applet.
Just my 2 cents.