Modularity adds costs in so many ways that nobody is winning. The cost of a modular platform WITHOUT the radio would cost as much of not more than the current price of the N900 with the radios included. You have to add bulk to the device to provide the mechanical ability to add the module to the device, the device itself needs certification from the FCC, ETSI, SAR, etc, as well as the device itself. Now instead of doing the certification on a single device, you now have to certify the device and each module separately. Do you plan on having a modular antenna system as well? The antenna design for each module will be different, certification will need to be done with the appropriate antennas for each band. You're not embedding the antenna into the radio module since this will likely be too small to allow an adequate element to radiate sufficiently.

Have you seen GSM modules? You can get a GSM module off the shelf that is roughly half the size of my Nokia E71. This is HUGE compared to the device you intend to use it with. Surely Nokia would make this as small as possible, but it's still a separate assembly that needs to be packaged to protect the circuitry and that add bulk, no matter how you slice.

Now you've got to set up separate manufacturing space for each module plus the device itself for test and calibration. You're adding to number of tests and test times by testing everything separately versus testing a single device.

Once you get through all that now you have the customer to deal with. Can you guarantee that the module will be placed correctly, having optimal contact to provide the best performance of the radio? Is the antenna connector robust enough to always provide optimal RF match?

You haven't decreased inventory at all, you've increased it. Instead of a single device with cellular radio, you now stock the device, multiple radio modules and maybe an antenna modules. Then you have to design packaging for each of your plug-ins as well. At the end of the day, nobody is paying less for a product such as that, and if Nokia were to product it you'd likely be saying "man, that thing is freakin' expensive, it would have been cheaper just to drop the UMTS radios on the PCB"... Exactly!