From what I can understand it's more difficult to develop apps for the BB than iPhone or Android OS. I think Dave's gone over this before and he'd like to provide a BB app, but it's just not that simple. He knows how many BB users the site has and how much we (I'm one of them) would appreciate a dedicated app, so I'm sure if and when he can he'll get one out.
In the meantime, I've actually found I can go straight to the site on my Tour and use the "media player" option to stream directly from the site. This is nice because I can find any feed and listen to it. The variety this affords has led me to set the Top 50 page as my browser's home page. I also have some of my more favorite feeds set up in moodio.fm.