Update: ROS on Arduino15 Mar 2013
It's been about 18 months since ROSserial was first released. Since then, ROS has undergone a number of changes, including a new build system, new message generators, and more. Unfortunately, this massively broke ROSserial.
Over the past few weeks, I've spent a bit of time fixing up ROSserial to use the new ROS build system, catkin. I've updated things so that message generation *mostly* works. I'm sure there are still a number of bugs in there, but it's now mostly working again and has just been released into Groovy as version 0.4.3 (currently on shadow-fixed, should be in the public ROS debian repos in a few days).
Recently, ROSserial has been moved to github like so much of ROS. Across the board, the move to github appears to have greatly increased community participation. Speaking of the ROS community, there is another ROS on Arduino package recently released: ros_arduino_bridge. Patrick Goebel of PiRobot.org and the Homebrew Robotics Club recently released this package which is primarily aimed at controlling a differential drive base. I've frequently heard people ask about running a diff-drive base using ROSserial, but the biggest problem is support for the massive nav_msgs/odometry message. This new package doesn't have the flexibility of rosserial, but definitely gets the job done for a mobile base.
Finally, ArbotiX drivers have had some minor updates and have been moved to catkin, and are released. Version 0.8.1 should be in the next set of public ROS debians.