The app is designed to be full-featured, and will let Rhapsody subscribers get easy control of the on-demand music service. Real Networks has described the app's operation on its Rhapsody blog:
The Rhapsody app is designed to stream music over 3G, EDGE or Wi-Fi networks. If approved, it will require a Rhapsody To Go account, which is US$15.00 per month. Spotify, a similar European music service, also has an app awaiting approval for the iPhone.You'll see a menu bar across the bottom of the screen, as is found in many apps. You'll have the option to check out the queue, your library, browse the Rhapsody library (ahem, all 8 million + songs of it), search, and settings. The app has all the functionality of the client, or Rhapsody.com, only super portable.
Apple already allows the Sirius/XM app, which is a different, but still a paid subscription service, and of course Pandora, Last.fm and other streaming music services.
All these apps are competition to iTunes, so the world will be watching to see if Apple draws the line with RealNetworks, or keeping a careful eye on the FCC, approves it.