We're all about the flying things here, but since Arduino robotics is also a focus I thought I'd pass this along. Pololu has just released a nifty two-wheeled bot based on the Atmega168. It's under $100 and looks really fun. Here's the description from the site:
"The Pololu 3pi robot is a complete, high-performance mobile platform featuring two micro metal gearmotors, five reflectance sensors, an 8×2 character LCD, a buzzer, and three user pushbuttons, all connected to a C-programmable ATmega168 microcontroller. Capable of speeds exceeding 3 feet per second, 3pi is a great first robot for ambitious beginners and a perfect second robot for those looking to move up from non-programmable or slower beginner robots."