I have used the mbed in the past for prototyping and I agree with Mark that this is a great compromise between functionality, low cost, and ease of development.

It also has a real 32 bit C programming model (none of the warts of a pic processor) that make us old C programmers feel right at home.

I think it has enough I/O capabilities to keep everyone happy (display/bluetooth/wifi/can/etc) while making the add-on boards affordable.

I don't know if the arduino shield model is the right one to pick for add/ons (it may be).  Perhaps put some base I/O on the board with options for expansion to keep cost down.

I'm ready to replace my rev2 hardware with something that has a touch display as I want a way to see data when driving.

I have played around with other options (including can bluetooth adapters) but the elm327 model of accessing data is pretty limited.

I give mbed my vote.

Phil H.