Trive wrote in about the robochild of a quite unorthodox LEGO/Arduino marriage:
It's a self-balacing robot inspired to the well-known Segway. Its brain is Arduino and that's why it's called "Arduway". Arduino is mounted on a hand-soldered board which connects it to two LEGO NXT motors and an Epson gyroscope.
Aduino Poet, the robot's creator and a student at the Politecnico of Milan, started a library called Arduino2Lego which aims to provide a simple API for the Arduino to talk to standard NXT sensors and motors. Currently, it looks like it only has an interface for motor control, but the project is open source and with any luck, others will chip in and add more features.
Arduway discussion at arduino.cc