I think the concept of multi-processor computing is a little esoteric and complex and the protocol would be too much for many.
I agree. This would not be for most arduino users. In fact, I would probably need help with the communication protocol myself as I am not an avid programmer. This would be for those with large projects where multiple arduinos would be a good solution. I myself am working on a large complex project. I'm using a mega and haven't run out of pins but with the complexity of the whole thing I often think multiple arduinos would ease some of that complexity. This could just be from my inexperience though.
Maybe offer SPI as an option as well. Much faster.
I have already thought about leaving it up to the user to select between serial, I2C, or SPI via jumpers. I always think choices are good. However,
What if you made it really simple to interface between processors
I agree. I strongly believe in designing anything using the KISS principal (Keep It Simple Stupid). And it would be much simpler to write a communication protocol for one bus type than many, but that takes away choices. I like the common memory idea and the mega does have external memory capabilities. I can see this(arduino as a shield) in itself is quite complex. Maybe a project for another time. Thanks for the input. DJ