Love it!
So, for the iphone I really like the search/filter feature, but there are time when I know I won't be using it for a whole evening. For the iPad this is no big deal (but still may be useful) but definitely for iPhone if under options there was a way to get rid of the bar because of the extremely limited real estate, each half cm matters.
Also, the way you have last.fm artist bio, what about lyrics in a similar fashion? That would be very very cool.
Lastly, you've built the app for portrait, and that is great. It is interesting because most of my apps go well in landscape, not portrait, but yours seems most happy in portrait. The most offending screen is the Now Playing, where all those buttons on the bottom should be on the right, and that would allow the image to be much much larger, rather than having everything stacked - which of course makes a ton of sense for portrait. I know that is getting pretty deep, the app is great, and having it optimized for landscape and portrait is something one does after a while but consider it. Maybe put it low on a list somewhere, but at some point it will get to the top.