We have released a serial slave program as an example project for the 3pi Robot. The slave program, together with an expansion kit, makes it easy for someone who is used to programming on another platform, such as a Basic Stamp, to build robots based on the 3pi, without doing any AVR programming. A hex file for the slave program is included with the library distribution for loading directly onto the 3pi, but you might want to add some commands of your own and recompile it yourself.
A master/slave configuration is also useful for expanding your 3pi to gain more sensors (for example, distance sensors or an IR beacon) or actuators, or just to get additional processing power so that slow computations don't interfere with a high-speed sensor feedback loop.
The slave code and an example master program are both included in the latest version of the Pololu AVR Library. The examples are based on serial communications functions that are now included in the library, so you could also use these functions to control serial slave devices from a 3pi or LV-168.
Please take a look and let us know what you think!