The more I've been thinking about it, the more I like the Beagleboard - lots of processing power, small size, lowe power consumption, plus a growing community of developers. I will, however, still have quite a few AVRs sitting in there, doing control.
Essentially, the AVRs will be working as the nervous system, the Beagleboard(s) the higher brain functions. Higher functions, such as OpenCV, Java Astrodynamics Toolkit, OBD-II interfacing... Fortunately, I have an OBD-II interface board that's been sitting away, ready to be used. I have to finish the last couple of things, like actually connecting the OBD-II plug, and adding in some optocouplers. Actually making sense of the data going back and forth will be interesting.
I still like the idea of having some neural network processors doing some work... and I do believe in using several processors doing individual jobs, rather than having one trying to do lots