I had a discussion with Devin Linnington and Hans Christofferson at a Silicon Halton meetup last week about using microcontrollers in robotics and remote controlled vehicle projects. Devin went home and came up with this amazing set of links:
Hack-a-Day - Great website for project ideas, thousands of random things on there
And related to that, I was looking for the super-cheap robotics platform but apparently that deal has expired. Found a cheap propeller platform though:
Sparkfun - Awesome resource for hobbyist-friendly electronics, also has tutorials for beginners (Hans, search 'motor driver' to find a ton of pre-built boards)
Solarbotics/HVWTech - Canadian company out of Calgary that has some more resources and really cheap solar panels
Seeed Studio - Chinese website that sells really cheap stuff. Also sells unlicensed bluetooth modules for less than half the price of N.A. retailers.
Open Circuits - Haven't used this site much but there should be tons of circuit designs you could use for your projects
...to which I added
Servo magazine is full of ideas: http://www.servomagazine.com/
The current online version has an article about using the Arduino to make robots.
There are lots of good project ideas in the Arduino Playground Wiki.