Here's an interesting stat I think tells a bigger tale: Onavo is a service for iPhone which claims to shrink your data as it goes in and out, and that service has been monitoring data coming back and forth from its users' iPhones. Spotify, as you probably know, is a music cloud service that recently launched in the US, and Onavo says that iPhone usage for Spotify has spiked since the US launch. In other words, Spotify's iPhone audience has jumped since the service was introduced to the States.
This doesn't mean that iPhone is the only way people use Spotify, as there's no information included about what Spotify's web traffic might be like, or the Mac app. But I think it does show that there's a growing trend of people listening to streaming music on their iPhones (something we've already seen from Pandora and other apps). Traditionally, streaming music has always been a web phenomenon, but obviously it's easier and more convenient to listen to music while out and about, and better mobile Internet connections have made that possible.
As usual, Apple's already at that party, with big plans for iCloud. That's not actually a streaming service, it should be noted: iTunes Match only allows for syncing, not full streaming. But this idea of music to and from anywhere is definitely a trend that's growing.