Similar to the IOIO board covered a few posts ago, I’ve been seeing several projects lately utilizing USB host mode on Android to communicate with an Arduino. This is a neat one which is able to send text messages to an LED matrix. It’s got a fail amount of detail and contains both the Android code and the Arduino sketch needed to get it up and running quickly. Here is a more general guide on using your Android device as a USB host, again with an Arduino.
As a bonus, the second link will get you to some how-to’s on USB communication with an Arduino. I’ll include the links here as well: