Perhaps I should post a bit more about the "idea"?
The modular development / hobbyist system would start out based around this development kit from FreeScale. It has quite a bit of I/O, both analog and digital, as well as many built in features such as CAN, LIN and others.
The vision is having something allowing one to work on prototyping of circuits and functionality prior to committing to a fully-integrated board, while also allowing something like a DIY ECU for those that want a one-off thing.
The plan, at the moment, is to design an interface board that can take signals from the above mentioned board, and route those to two or more separate locations which would accept Arduino Uno Shield type boards. Each location on the adapter board would have its own set of I/O dedicated to that location, or, more likely, shared with adjacent locations, to help keep stack height down; we have already discussed and would like either 2 x 2 or 2 x 3 layouts for this adapter board, where I/O could be shared across a column or a row, within some limitations, of course. I'm partial to the 2 x 3 configuration with 2 rows of 3 positions sharing common pins, since the third position could hold an SD card Shield and log everything that happens on that "bus". Heck, an Arduino processor itself could be mounted and provide some functionality such as something that needs pre-processing prior to data being given to the ECU.
The tuning software we are developing allows assigning almost any ECU I/O function to almost any pin at run time so pin clash is not so much of an issue, but being able to selectively route pins on the Shield to pins on the interface board may be useful in certain situations.
The types of boards we would like to have available are things like I mentioned above: a shield to drive two ignition coils, preferably as low-side with external IGBT or logic level (LSx type and similar coils) to keep noise out of the system; a Shield to drive two fuel injectors; Shields for interfacing to analog sensors such as ambient and manifold air pressure sensors, throttle position sensors, temperature sensors and the like; Hall-effect or VR sensor interfacing for crank and camshaft position sensing; data logging; motor control, think throttle-by-wire or idle-speed control; wheel-speed / loss-of-traction detection or control (maybe combined with throttle-by-wire?); CAN over wireless; ECU programming (?).... 
The premise being that one has a starting point and can add Shields and necessary to achieve the desired level of functionality, up to the capabilities of the main board.
We are serious about getting something done, but we are also hobbyists doing this on our own time and with our own dime. The goal is having open-source software and firmware- the licensing is BSD-3-Clause, but we are supportive of those who might want to design their own hardware and want to commercialize their product: we have a member designing his own ECU which will be capable of accepting the Microsquirt Module and a forthcoming (and proprietary to the developer) module based on the MPC5634M processor used in the above mentioned FreeScale TRK kit, and the coders are working to ensure compatibility between the firmware and the module.
We understand the processor we are using is not an Arduino-related unit, but appeal to the Arduino community because we know this is a strong ecosystem and with the newer processors available to the Arduino community, the Shields that are useful to us, could be useful to the Arduino community; I don't know how many sites I've waded through looking at "Arduino EFI" ;).
And I know it might sound like we're asking for something for free, but we are not placing any restrictions of hardware with regards to licensing. If you want to develop it, feel free to do so and if you want to charge for it, likewise. We're asking for help, not hand-outs.
Kirk