It turns out that iTunes sharing is just HTTP, as pudge observed. The problem is that you can't just make a single GET to download a file. You need to get a session ID, figure out which database you're accessing, and learn the unique ID of the song you want to download. And the responses from all these requests are DMAP files, a hideous binary pseudo-XML-with-types that's a turd to parse (I know because I've written three turds of parsers so far). The only thing I don't have sussed is authentication, but I believe that's easy to add.
I've got Quicktime and mpeg4 demoing the command-line shell for connecting, searching, and downloading files from someone else's iTunes. I'm not releasing the source, though, until I talk with Tim O'Reilly and Apple and figure out whether I'll be getting my arse sued off