Difference of perspectives
I think (almost) everyone has made a good point or two on this thread. The various positions of the posters seem to be largely driven by their perspective on time line. The mobile solutions folks are looking at trends in the field and thinking in the 2 to 5 year time frame---an eternity in high tech. And you are right; the trend is towards mobile computing (maybe not phone, but something similar). The carPC folks are looking at "what can it do for me now?" and not terribly interested in where the market is going. You are right too; no small mobile platform will come close to the power and customizability (is that a real word?) of an on-board system.
My suspicion is that you die-hard hobbyists out there will always be the vanguard on exploring the possibilities of in-car technology. But, as the really useful/valuable/cool applications you guys develop become obvious, the big boys are going to figure out a way to repackage it into smaller, more easily installed units for the mass consumer market. Because there will always be people like you pushing the envelope on high-tech apps, there will always be a need for dedicated on-board systems. Because there will always be people thinking about how to make a buck off the deep-pocketed, technologically un-savvy, there will be a strong push to "productize" the great stuff being developed here. Integrating into the rest of their "high-tech" lives will inevitably mean moving the big sellers to mobile platforms.
My 2¢ ... or maybe 25¢.
PS - I will, however, give a BIG thumbs up to cloud computing and all that it implies!