[follower] prototyped a 2-line external display for his Nexus One using an Arduino with a USB Host Shield, and the Android Open Accessory Protocol. There are two basic software pieces at work: an Arduino sketch that handles displaying data sent from the phone, and a lightweight android app to detect the presence of the external screen and send data to it. As shown here, it diplays the time and the beginning of the most recently received SMS message.
This project coalesced from several other things [follower] had been working on with regards to USB accessories, background services, interfacing with the Arduino and handling SMS messages, so it’s modular and open-source. If you’re interested in mashing up microcontroller projects and your android phone, there’s plenty of stuff in this project to help you get off the ground.
As hacks go, this is very much a “because you can” sort of deal that’s designed to tie a bunch of cool things together. You’re unlikely to catch us carrying an LCD and breadboard around in our pockets any time soon, but it paves the way for some potentially fun phone accessories.