I know I'm a little late on the scene, but to go along with what cyclegadget posted, do you know about either the Arduino ADK or IOIO? The Arduino ADK is an modified Arduino Mega designed to interface with Accessory Development Kit (ADK).
IOIO a purpose-built microcontroller board, somewhat similar to Arduino Uno, but purpose built and program to interface with Android devices (1.5 and higher). Of the two the IOIO is cheaper (~50 USD) and would probably be much easier to interface with multiple individual Android devices, but it's designed to act as primarily an accessory to the Android device(s) (i.e. you don't program the IOIO directly, instead code is run through an app on your smartphone/etc... and instructions are sent to the IOIO) and not setup to do much without an Android device controling it.
On the otherhand, the Arduino ADK is a fully functional Arduino Mega that's been changed to interface more easily with Android devices so it can also run as a stand alone microcontroller. However, you likely still need to program a custom sketch for your project and its more expensive (~80 USD) than either the Arduino Uno or the IOIO. Although it has more I/O and its processing power and storage space are significantly better than either of those boards.