I don't agree. I have a Samsung sch-i730 with 1XEV-DO and 1XRTT connectivity full time. I also travel to Yellowstone National Park on a very regular basis. When I'm inside the park, my phone is dead as a doornail. My XM radio is humming along just fine with a few minor fades on the north end of the park where towering mountains obscure my view to the south.
When I'm in a good EV-DO coverage area, the phone can do pretty high rate transfers. More than adequate for streaming audio content. The bad news is that most of the nation is still under "National Access" aka 1XRTT with a top throughput of 144 kbps. On average, you get a bursty 4 kbytes per second with 1XRTT. It's going to be a long time before the cellular infrastructure can support streaming audio everywhere the way XM does today.
I have 5 railcars connected with 1XRTT radios (Kyocera M200 modules). They also have GPS. The consequence is that I can create coverage maps for Verizon National Access. There are lots of areas with zero coverage. That necessitated creating a "store and forward" protocol for the railcar data. I keep a big FLASH disk on the car with ability to buffer data readings while I'm out of coverage. Once back in coverage, the stored data is fairly rapidly flushed back to the server and deleted from the FLASH disk.
I wasn't referring to cellular Internet access. It would have to be some other wireless method, perhaps satellite, like Sirius and XM. I predict those sat radio companies will try to stave off the inevitable by offering other content so others in the vehicle can access news, weather, sports, local radio stations, etc.