It is a function of the mail processor. The celluar radio(s) are totally separate parts. They can be powered on/off independently of the processor. And all they do basically is handle talking to the cellular network(s).

The Internet Tables and now the N900 aren't really designed to be powered off normally. They run all of the time, but they do have the several power saving modes. Those power saving modes go all the into the ARM processor. The ARMs have the ability to power virtually all of the core in max power saving mode. The only only things running are the wakeup circuity (i.e. power button) and a real time clock which can be programming to wake up the processor at a set time. (Although I don't know if the NITs and N900 go that far down in power save modes)