I'd also accept any suggestion about the microcontrollers, maybe a cheaper or more power efficient one?
Get copies of the last Sept/Oct Elektor magazine, great series on protocol for comm's between processors. Started in January, but last couple are where it really came together.
Also, local wallwart for each board unless you want to send out like 10V and use local switching supplies to power each board efficiently.
You don't give enough details about your system, like how much power each board is going to consume.