Actually, no you wouldn't - there is a Bluetooth feature (probably designed for cars/automobiles and other fixed locations) called Remote SIM Access that allows a cellular device to gain access to a network using the SIM in a paired device. Obviously both devices can't access the network at the same time which is why it's ideal for cars, but could work in the case of a "N900" so that the user doesn't incur dual contract charges.

Whether it is workable or not is another matter - if the N900 is using the cellular network for data access the N900 then needs to be able to handle voice calls as well since the phone is no longer connected to the network.

Perhaps a SIM slot could be included in the N900 as well, and if it's populated with a valid SIM the N900 can use that SIM to connect to the cellular network and if not it uses the SIM from a paired device.

It's almost certainly more trouble than it's worth to be honest, and we may be better off with the Bluetooth modem trick due to it's simplicity... personally I'm not interested in seeing mobile phone functionality in the N900, the device would be too big and probably lead to more side talking embarassment for Nokia.