At first, I'd turned on Ogg Vorbis sharing, but the software build for the NSLU2 is the last stable version, 0.23, rather than the current in-CVS version, so it doesn't have support for Ogg-to-MP3 transcoding. I thought iTunes would still be able to handle the Ogg files, since I've got the Ogg Vorbis plugin installed, but it looks like that only applies to files in the library, not streamed music. As most of my CDs are ripped in 192Kbps Ogg format, this is disappointing. However, I've got a lot of tracks from eMusic that work quite well with this setup.
I'm disappointed that iTunes doesn't deal with the shared files very well. I can't rate them or add them to playlists, as far as I can tell. There is a way to specify "smart playlists" on the server, but it involves a bit more coding than I'd like. In the meantime, I'll need to reorganize my music folders to move all the true music into one tree and move the podcasts and spoken word into another tree; that should allow me to setup a query that will only return music, so I can do shuffle playback without worrying about having a bit of some comedy act or a random podcast popup.