The folks at the have posted a brief hands-on review of the app, and it sounds promising. The ability to cache radio stations not only allows you to listen to the radio when you don't have a network connection, but also increases your iPhone's battery life, as streaming over 3G or WiFi uses more power compared to playing a file stored on the device. Last month, online music service Rhapsody .

Slacker 2.0 will enable you to cache up to 25 stations, although it's not yet clear how much music or disk space will be available for caching. It took the reviewer between 15 and 20 minutes to cache six stations over Wi-Fi, and it's unclear whether caching will be allowed over 3G. Unfortunately, due  to the iPhone's background app limitations, station refreshes must be performed manually by the user, but Slacker will give you a refresh reminder when you begin to run low on content.

Offline playback won't be part of Slacker's free service, but will be bundled with its Slacker Radio Plus package, which includes station caching, unlimited skips and commercial-free music, and goes for $5 per month or $48 for a 12 month subscription.