I am heavily into robotics and am looking at Arduino to be a new controller for my robots. What I see missing is a shield for bringing the Arduino’s I/O and power pins out to a form more easily usable in robotics.
This shield would bring the I/O pins out to three pin headers with power, ground, and signal lines ordered for attachment of standard servos. It would also be real nice to have external power connections available for an external battery supply (jumper selectable between the Arduino power and external power).
A socket and connections for a v2.0 Bluetooth dip module could be added for wireless connectivity. 
The rest of the board space could be dedicated to a solderless breadboard.
This sort of shield would make the Arduino great for robotics control use. I really like the concept of an Open Source hardware/software platform for embedded control that has tools for Linux. 
With this shield, I think many would start using Arduino for robotics.
8-Dale