Modu - Five User Interface Advances to Watch in 2008
Modu's modular, multi-function handset was one of the most talked-about innovations to surface at Mobile World Congress. The Modu system comprises a tiny, 1.3-ounce core module that integrates with a series of "jackets," effectively enabling users to transform their phones into different kinds of mobile devices depending on individual or contextual needs. The core module's domino-like UI is extremely simple and user-friendly, with a micro-USB interface and a proprietary port on the bottom.
Modu demonstration video:
Comments
Sorry Modu, but you're years too late... Seems there was this device way back when, sorta semi-vaporware called the Omniplayer. Take a look...
http://support.gateway.com/s/tutorials/Tu_847209.shtml
The idea was that it started out as an .MP3 player, had two MMC slots and the interchangable faceplates had ROM's in them so that you could change from audio player to PDA, to GPS, to whatever they might eventually offer. I thought it was a great idea, back then... But try as I might, I never saw one IRL.
So Modu, good luck with that.

