I dont think so. I think the most you will see is a system that resembles a PC in that you might have internet, email, etc. Running Windows is reliable and great if you know what you are doing, but for the average user, I think there would be too much potential to screw something up.
In the future, there might even be so much features and integration between the multimedia system and the car that the carpc hobby will slowly die because having a carpc may not give as many functions as the stock system. Here's a video of what I mean:
(I think that's the right video...youtube doesnt isnt loading here at work)