Michael Ferguson and Adam Stambler have released the rosserial library for Arduino.
This is a new library for connecting the Arduino platform to ROS, allowing an Arduino to directly publish and subscribe to ROS messages. There are also demos ranging from controlling servos to using an Arduino and rxplot as an oscilloscope to reading temperature sensors into ROS.
In addition to support integration with the Arduino platform, the rosserial library provides a general point-to-point transport for ROS communication over serial, which is intended for hardware that cannot support the full ROS TCP/IP network stack. This library can be used to easily integrate a wide-variety of low-cost hardware into ROS.
For more details and downloads, visit the Ros website.