Arduino Mega ADK - available from the Arduino Store and Radio Shack $84.99
DIY Drones PhoneDrone Board for Android $99
mbed has a few solutions...
Microchip - Two boards are available, the PIC32 DM320412 (not yet for sale actually), and the PIC24 DM240415 $79.
Modern Device's Freeduino USB Host Board $79
RT Corp - RT-ADK&RT-ADS $389.03 (31,500 Yen)
Seeed Studio Seeeduino ADK Main Board $79
Sparkfun IOIO for Android (beta support) $49