Well, it's finaly here. Songbird have release a Proof-of-Concept release of their mp3 player which is based on Firefox.
I've only had a quick look so far, but apart from the integration of a web browser into the application I can't see anything to write home about that couldn't be achieved through a good Firefox extension.
Hopefully I'll find more when I have more time to play with the service, but at the moment I don't think iTunes has anything to worry about.
