Robotics is an excellent idea.
My plan is to hook the Pi up to a Propeller microcontroller from Parallax Inc.
http://www.parallax.com/
A simple serial link can be used either through a USB/Serial adapter or just direct connection from the serial port on the Pi's GPIO pins to the Propeller pins.
The Propeller then has 24 pins that can be used as:
1) Simple GPIO bit banging
2) or PWM drivers for servos, ESc etc. You can drive up to 24 servos from the Propeller.
3) Analog inputs and outputs (needs a couple of resistors and capacitors on the pins to do that)
4) SPI and I2C interfaces to accelerometers, gyros, or whatever devices.
5) Provide VGA or TV graphical or text output. (No HDMI required to talk to the Pi)
6) etc etc
The Propeller has eight 32 bit processors on board all of which can use any of it's pins so its easy to do many of the above at the same time and there is no worry about running out of processing power or messing with interrupts.
The Propeller can be programmed in C or it's own "Spin" language which is very easy to use. There is a lot of ready made libraries for all kind of interface functions available.
Just now I'm working on getting the compilers and IDE for the Propeller working on the Pi so that a Pi + Propeller combo is a stand alone, self hosting, development environment.
Memory in C++ is a leaky abstraction .