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Kompoz is the latest to launch an iPhone-optimized service. The community that lets you collaborate on music now grants you optimized access on the iPhone. This won't officially launch until Friday, but you can still check it out now.
The main components of the mobile Kompoz site let you search for music, check out genres, channels, and users. You can listen to the music that's been uploaded for various projects, leave comments, track comments, and stay updated with various projects taking place on the site. There's also access to your projects that you've created--see who your collaborators are, and see the online status of each user.