Lots of people in the Raspberry Pi forums are working on this. I bought three Pi's to play with, and am progressing a little at a time, adding functionality necessary to make a Pi a viable robot controller. (I'm a mechanical engineer, not a software guy.) I hate to advertize my future plans, because the web is full of people announcing big plans with no follow through. But, to date I have been successful in using the Raspberry Pi to control a Jaguar and a servo using a joystick as an input device. With no additional hardware, you can control up to 8 PWM channels. Adafruit sells a $15 accessory board from that lets you add 16 PWM channels at a pop. And it's only a matter of time before other inexpensive boards hit the market allowing for analog and digital I/O.
Video of joystick control here:
http://www.youtube.com/watch?v=0ODQY2mDYR0