• The N800 has very dynamic power saving features, which means that the actual battery drain can vary dramatically depending on what it's doing at any moment. For example, when just sitting there on the desk with the backlight it can last 9-10 days. On the other hand, when running the internet radio it'll be a noticable drain on the battery after maybe two hours. If you execute a program that just loops the CPU 100% then it'll drain the battery so fast you can see it.
  
• The battery meter tries to indicate this ambiguity: Right now, when I tap the battery meter it shows 'When inactive: 10 days. When in use: 7 hours'. Obviously it can never guess completely right, because there are just so many things you might do in the next hours that will completly change the original prediction.
  
• Wi-Fi can sometimes drain the battery in an unexpected way, but this isn't too common. For example, I have a wi-fi router which somehow prevents the N800 from going properly into wi-fi power savings mode. It'll half the battery charge in two hours. On the other hand, the wi-fi at work represents a very light load, I can keep my N800 on that wi-fi for probably 2-3 days without draining the battery. I also seem to see a correlation between the speed of the network and battery drain: The faster the network, the better. A low-latency network is also better (that's the time you seen when you 'ping' a remote address). I'm guessing this is because fast = the N800 can do the networking in quick bursts and then go directly into wi-fi power savings mode.
  
• Sometimes certain SD cards will drain the battery unexpectedly. This has been reported several times here on ITT. I have no such problems with any of the cards I own though.
  
• File system crawlers can sometimes be the cause of battery drain. I have twice had the N800 run out of juice over night. I disabled the built-in metalayer-crawler in my N800 and haven't had any reoccurances. I have no proof yet, but my gut feeling is that the crawler was the culprit.
  
• Another time a runaway process emptied my battery. Turned out it was because I had started a program in osso-xterm, and instead of quitting the program and exiting osso-xterm I just stopped osso-xterm with the 'x' in the corner. Not a good idea.. some experimenting showed that this would leave the program I had started as a runaway process.
  
• After the above trouble I now have a CPU monitor (I use load-applet-run) installed, so that I immediately notice if something's abusing my CPU at unexpected times.

Lastly, under normal use my N800 will last for days before I really need to charge it. But in practice I charge it a bit most days, because I can (and maybe I'll be using one of those no-good networks later, when a full battery is a good thing.)