I've noticed that if my N800 resets itself I automatically lose half the charge on the battery. For example this morning I had a full charge and was playing a few podcasts on the way to work (approximately 1:15 commute). At about 45 mins in media player choked on an MP3 and when the n800 restarted that battery indicator was showing battery low.
I tuned it on a few minutes ago and found I now have four hours left active. No other applets or applications were running and the n800 was set to offline mode.