Mugen Power Batteries have released a bigger battery for the N900 that should def give you a few more hours use.

http://www.mugen-power-batteries.com...tery-door.html

I have had my N900 replaced once already because the battery did not last longer than 3 hours (with minimal use) and even though the handset was faulty, I did consider a bigger capacity battery.

Bit expensive and will 'beef' up your tablet but might be worthwhile for the heavy users amongst us.