Maybe I don't understand the concept correctly but, why not run the web server in the car? If the hardware/software is smart enough, it'll detect when you have connected to the car's network and maybe prompt a notice on your iphone? Then you just load up the flash/php/whatever page and do your thing.1. Obviously, the downside is that when not connected to the net the user would experience interruption of service.
2. Right now, many users do not have internet data service inexpensively. However, in the next few years, that will be less and less of a problem.
if you have access to the net, the web services can utilize those. If not, you still can access the basic functions of your car.
The problem I see with being totally "headless" and having all the control in your smart device is the case where your wife gets in the car and she wants to do something, but left her phone somewhere and she can't find it. Also, what if your phone battery dies mid trip (shouldn't happen if you have a car charger, but heh...)? I think the idea is good, but I still think there needs to be at least a very basic system in place where you can control the system without your smart/mobile device.
For me, the optimal integration happens when the carpc system is smart enough to completely integrate with your mobile device. Bluetooth proximity detection, UPnP file shares, synchronization of gps bookmarks, etc are some of the obvious things. Then you can still use your 7" Touchscreen without having to squint at your iphones 4.3".
It should also be noted, IVI interfaces differ from the mobile interfaces found in phones and MIDs. Those interfaces are still designed with "captive attention" in mind. IVI interfaces cannot demand captive attention by nature. They are close, but the differences do matter. Thus anything that your run on your mobile device in the car will have to be designed for that purpose. You can still get away with running mobile apps in IVI environments, but it's probably not going to be optimal.
Just my two cents...