Ok, charged my battery up in a friend's 5800. Put it back in the N900 and it still won't start up. Nokia logo screen appears but after a while it goes away and it's just a blank screen (i.e. how the screen appears when the phone is off).

If I plug the mains charger in I experience the same problem, logo screen comes (washed/dimmed out), LED is off. Then the LED turns yellow for a while before the phone turns off and on again, repeating that process.

I don't think the battery is the issue. Anything else to try or is the phone dead?

Edit - I also tried my friend's 5800 (fully charged) battery in my N900 and it didn't start up with that.