I am aware of the s3 fuse project(s) and have played around with that, but it's clunky and doesn't really work correctly to begin with...
I am thinking something along the lines of Subsonic getting the file list from s3, but when a user listens to a song we send them a private link to the file in s3, so the bandwidth would be on s3 not the server hosting Subsonic.
If this is feasible and something you are interested in I would be willing to contribute to developing it.
I love Subsonic, thanks Sindre!