i AM often away from usb ports...
cause i use my n900 at places "as a pc" where i even dont have a subnote, f.e. when with the boat. or in the garage. or at my relatives. or or or...
i often load the battery in my car when i drive to such places. but it would be really fine to be sure that one can use(!) the n900 for more than 4 hours before battery goes low.
And when battery is low, the n900 refuses some times to load in the car or even at home - so a low battery is a really annoying thing.