This is honestly the first BlackBerry app recently that's made me almost wish I hadn't traded my Pearl for a G1. Sure, I already have a Slacker Portable player, but it would be super awesome to have the same stations playable from my phone whenever I'm somewhere and don't have my Slacker portable with me.
The BlackBerry app (available for free), allows you to stream your favorite Slacker stations from anywhere. But the killer feature is the ability to cache stations. This essentially turns your BlackBerry into a Slacker portable by allowing you to listen to your stations even when you don't have an active network connection.
Laptop magazine even gave the BlackBerry app their Best of Mobile Entertainment award. Check out their review and video tour.
Unfortunately, I can't try it out since I don't have a BlackBerry anymore. Here's hoping they launch an Android version soon! :-)