I expect this has been mentioned already (this is a long old thread) but I've been testing a scenario yesterday and today which looks to have identified at least one cause of rapid battery drain.

Basic situation:
My N900 has the latest firmware
3G is on
wifi is on but not associated with a network
bluetooth is off
facebook and rss feed widgets are on the desktops (2 in use)
location is on
2 IM accounts active
3 email accounts active

Yesterday:
N900 off and charging overnight. Battery fully charged when switched on at 08:00. Throughout the day it mostly sat on my desk at work, with light usage of 30 mins music playback on the bus, 2 or 3 phone calls, a couple of texts, reading and deleting a few emails and picking up a skype chat conversation (I wasn't chatting on the phone, it was picking up the conversation I was having on my laptop).

By 18:30 the battery was red and I got the low power beep. Not that great really...

Today:
Same starting situation and time _except_ I disabled the 2 IM accounts. Similar usage compared to yesterday (actually a few more phone calls), minus the IM chats of course.

By 17:30 today - approx 90% battery still remaining I'll keep the phone switched on and measure the eventual life of this charge with just the disabled IM accounts being the difference.

Cheers,
Alex