I flashed my n800 several times. I always install a lot of stuff, so I couldn't track what was killing my battery even when in offline mode and locked. By reading this thread I came to realize that the problem could be that I was using virtual memory (swap on my mmc card).
Once I stopped the swap, the battery keeps the n800 for days.

Thanks for the hint! I'm seeing quite disappointing battery life on my n800 that's just a few weeks old, and I guess this might have been it. I do have a swap partition (which is hardly ever used), and I'll try without it.

Anybody knows if booting from SD card might also affect battery life?

Since the first days of my N800 I had a swap file on the internal SD card (original card from Nokia). No problem with the battery for months. Just recently it started aging. Already ordered a cheap chinese replacement from ebay.

Could it be related to the new kernel and not the aging of the battery? did u try lately without swap?
In my case it was the only thing I did to regain the life of the battery.

I'm starting to believe this problem is coming from the new firmware.. probably the one that was supposed to fix some corruption problems in SD cards.. maybe at the cost of draining the battery. My N800 was fine till a some 3 weeks ago and the battery used to last for several days. Now it can't even last for 24 hours! Aging doesn't really make sense.. batteries don't age in a week!

if the battery dies in less than 24hours then find out what app is hogging all the CPU.

I don't have the new firmware yet, and I don't have any CPU hogging programs running. Since about 4 weeks ago my battery has only about half of its capacity. 2-3 hours browsing, compared to 4-5 hours before.

Soon I will get a cheap chinese replacement and will check and report its capacity here.

I tried now leaving the n800 without charging connected to wifi and running pidgin overnight and there was still battery for another 5 days.
So for me the fix was removing the swap option.
Also I am very happy to know that I can now be online 24x7