Only problem with that is that means

a) either paying for a seperate cell phone plan

b) tethering the N900 to seperate cell phone plan (which would consume both batteries)

c) switch out the SIM card, but that means opening the N900, taking out the battery (which is hard!) and removing the SIM. Putting all that other stuff back in the N900, then open up the other phone, put the SIM in that one)

Sadly outside of carrying two devices (which I did for a while before the N900 arrived, n800 and tethering it to a cell phone) there's no way would normally want to use the N900 completly like you did the n800 unless you have an alternative power supply nearby (AC, DC, power pack, spare battery, etc..). Simply because like I said, if you run out of juice, there goes your phone capability.