Casey, I'm not convinced of your belief that Pandora and the carriers could cozy up. Pandora is a friend of the carriers because it's a (relative) data hog, but that means they'll resist any urge to let that traffic bypass the meters. It's a good potential income generator via overage on capped plans. The only way a "bypass partnership" would happen, in my opinion, is if it cost the consumer more money. And I certainly don't see Pandora easing up on the 40 hour streaming limit. If anything, it may gradually be reduced to get more people into subscriptions.
Spotify just debuted in the US and I've been giving it a test drive. Right now, free accounts have unlimited listening, but that's only for the first six months. After that, it's capped at 10 hours a month with 5 replays of any one track. The freedom is the bait, and the capping is to drive people who suddenly find the service worthwhile into a subscription. Right now I enjoy being able to listen to entire albums and user-generated playlists, but come time for it to be capped, I'll probably uninstall it.
The question is, how much are people going to be willing to spend to listen to music on the go or at home? People have already balked at Sirius/XM's prices, and Pandora, Spotify, Rhapsody, Grooveshark and others are all dependent on subscriptions to stay afloat and so far, few are biting. Pandora's paying subscriber base, out of over 75 million active users is something like 700,000. Less than 1%.
Free listening (with ads) is the model they get their revenue from, just like radio. The difference is radio has never had a limit on how long you could listen. It's that reason that I don't believe Pandora will really affect music radio listenership much.