Spotify has launched an API service that could have some really interesting results for both iOS developers and users of the music-sharing program. The API allows access by iOS developers to all of Spotify's 15 million music tracks, which can be played or used as they see fit.
There are some interesting ideas to play with here. Developers could make a game that uses Spotify tracks, do things like match up music tracks for recommendation, or hook them into any number of various app types out there. There are a few catches, however. First, anyone who uses the service must also be a Spotify member (though there are quite a few of those around at this point). Second, the current API only allows for "noncommercial" use, so free apps only. Spotify is working on a way to set up deals on for-profit apps, but it's only for free apps so far.
If you're a developer who wants to play around with the interface, you can find libspotify right over here. It'll be fascinating to see what comes out of this, especially if Spotify works out a way for commercial developers to dive in and use their content as well.