The latest update to the iPhone app for online radio service Slacker, which offers a combination of personalized radio based on particular artists and curated radio stations, just became available in the iTunes Marketplace.
Caching is particularly useful if you spend lots of time out of the range of a wireless connection, like in a subway tunnel or on a plane. On the iPhone, cached stations will start with about 100 songs, but as you listen to those stations while connected, the app will gradually download more songs to the cached version of the station on your device, for a maximum of about 500.
The Slacker iPhone app is free, but to use station caching you'll need a Slacker Plus subscription, which starts at $4.99 a month. That's about half of what you'd pay for the iPhone version of , which offers on-demand playback of any song in its catalog of 9 million and lets you create and manipulate your own playlists.