Hi,
Ok its only my Radio Controlled Race car, but it is my main race car and not a toy RC Car.
So far I have built a traction alarm, interfaced to the receiver to drive the servo and build a g-force data logger.
Details on my blog - http://rcarduino.blogspot.com/
The main project I plan to tackle is building a traction control system, I am part way there in already being able to sense and measure the degree of wheel spin and also to capture the receiver signal and then output an arduino generated equivalent - to control traction I will need to put these two parts together.
If your interested in my projects of the other possibilities of combining a hobby quality RC Car with Arduino have a look at my blog, there is a page of suggested projects that might be interesting to some but which I have no immediate plans to tackle.
Duane.