This is a really fun project. I did something similiar for my C# class earlier this year. For class, the project used a slightly broken toy RC car as the basis, some arduino stuffs to control the toy car, bluetooth on a laptop for communication, and an xbox controller on the laptop. In general, I found there were mainly three parts to the robot part of the project: hardware, control, and communication. For example, my car had two motors, one for each side. To control these, an arduino with the adafruit motor shield was used. To communicate, bluetooth was added to the arduino. A netduino could be used just as easily as an arduino. I didn't use a phone as a relay. It would be really fun, but I would probably leave it for last. Sparkfun is a great place for hardware. If it helps at all, I'm working on describing the project, here. (It's a work in progress).