Wireless in Cars versus Mobile Phones in Cars

I saw this on FierceWireless Today:
Gartner Says Wireless Connectivity to be Main Focus for Vehicle Manufacturers by 2012

This is intriguing - wireless in cars. It should give carriers new subscribers, which is good. The ARPU will be pretty low - but does ARPU still matter when you're talking about devices that aren't phones? For reference, the Amazon Kindle II's ARPU for Sprint is something like $2/user.

However, I wonder: does it make more sense to put a wireless app, the infrastructure, the SIM, etc, into a car, or does it make more sense to use everything the mobile phone the passenger already has to do the same thing? Maybe through bluetooth (or some other way) the car could communicate with the passenger's phone, and piggyback on the existing infrastructure and handset ownership.

Either way, a subscription fee for wireless in your car when you already have a wireless device in your pocket seems like a pretty hard sell.