[lazyjones]

Absolutely correct. This is why such a device should isolate the hardware components used for communication from the main CPU/device, consider the former "hostile" and communicate with them using a simple, safe interface (like USB or serial). Using a throwaway external 3G/LTE adapter (USB) would be even better. This way, a compromised baseband processor or SIM card cannot access the host's memory (using DMA like in current smartphones) and as long as the host uses secure encryption, it can still communicate securely (but of course the device will be detected and identified).

[bergie]

I think this is essentially what N900 did. The GSM part was handled as an external modem so that people would be free to have root on the rest of the device.

http://flors.wordpress.com/2009/08/27/software-freedom-lover...