I'm an "early adopter" myself.
But as I was reminded in my early days of programming that by the time a jock is sick of a song, the audience is just becoming aware of it. It's a reminder to me to "slow my roll" when thinking of a "majority" or a "movement" that may just be my circle of co-workers or friends.
So when I think about a lot of the markets I've worked in that are smaller, blue collar markets, and the economic picture, I see a slow adoption of mobile internet radio outside of the most urbanized or educated & upper income markets.
We've seen a move by several wireless providers to charging for data past a certain cap. With all costs of living - fuel, food, etc. rising and a less than stellar job market - there will be fewer people who can afford to have it all - especially when there's only so much time to consume all of it. The budget has to be adjusted somewhere. That might be cable, that might be wireless data, or subscription music services.
There are huge portions of the country (I have personal experience with) that only have access to WIRED broadband in the past few years, and the speeds are less than ideal. To look at wireless coverage maps, there are huge areas where you can't listen to mobile radio reliably. You'd have to see a greater investment in an infrastructure that may reach relatively few paying customers - someone has to pay for that.
Now, if our nation developed a serious plan to invest in infrastructure - it's possible some of that cost would be shouldered by the government. I'm not seeing that happen in the current political climate, or until there's a significant economic improvement that would encourage more private investment in less than urbanized areas.
The ideal for internet radio is to be a push button appliance. You get in the car, you tap Pandora, or whatever "preset" stream you like, and it starts playing. For that to work from a hardware standpoint is getting better and in more autos every day. That's not the difficult part in the long run.
The difficult part is making it a service as easy to use for the "average listener" as Sirius/XM, or your local FM stations - achieving reasonable coverage across the country, and keeping that within a cost point that doesn't pressure the budget of the majority of the public.
I'm guessing the "stars won't align" for webcasting till the better part of a decade. This isn't even taking into account the royalty issues already raised.
Coming from the terrestrial side of things, I'm not scared of the advances, and I believe great local brands can survive on new platforms. My business plans definitely incorporate new media and streaming. But I'm not afraid my FM licenses will be worthless anytime soon.