This in car internet radio unit was revealed last year at CES and BMW were rumoured to be the first to offer a factory fitted option. However it still seems a way off for most car manufactures to offer Internet radio as an option. It could be current mobile phone networks are against it due to current bandwidth constraints.
A DIY method is possible TODAY using a smart-phone with an unlimited bandwidth 3g contract and connection and then either use a small FM transmitter to re broadcast the signal to your cars FM stereo (Some Nokia smart-phones have a built in FM transmitter for this reason) Some car stereos also offer a USB input or a bluetooth connection.
I have experimented myself and it can work while traveling in the UK where most major motorways have good 3g coverage but if you go off the beatern track it can keep disconnecting or the stream does not buffer correctly.
The future of mobile phones is extremely exciting beyond 3G called LTE (Long Term Evolution) using MIMO technology (multiple-input and multiple-output) using two or more aerial antennas and this will make posible bandwidths up to 150gigabit per second. I have seen demoswhere full HD movies have been streamed to a test mobile. The limiting factor for mobile phones will be battery life the more power and features and higher bandwidth is going to use more battery power. However if this is all possible in mobiles phones then its all possible to build in to a car.