FORTUNE 1000 company UTStarcom announced at the 3GSM 2006 their MobileCard technology, which is world's first technology independent, modular mobile phone.

The UTStarcom MobileCard is the size of two SIM cards and less than 3mm thick. With this technology, a regular phone is reduced to a "shell," which typically consists of a housing, keypad, LCD, and speakers. Users can simply insert the module into the "shell" to have a complete phone.

This technology sounds similar to what Willcom is offering in Japan with their PHS W-Sim module.
UTStarcom's MobileCard technology was developed to create significant cost savings for mobile phone manufacturers. It is designed to eliminate the need for complex RF and baseband design during the mobile terminal development.
More details in this UTStarcom press-release.