The Nexus 7 is the latest and greatest Android tablet and one brave soul has hacked it into his 2008 Dodge Ram dashboard. Android Community calls it "quite possibly the most amazing in-dash entertainment system" they've seen. The tablet can be removed from the dashboard, but while it's docked it can playback media right to the car's stereo.
Unfortunately, the Nexus 7 doesn't connect to the web on its own, so one would need to tether the device to a smartphone or mobile hotspot to stream web radio. But that's no different than the slick OEM options rolling out from Ford, GM, Toyota and other automakers.
Android Community has more coverage and a video walk-through of the hack right here.