I'm having the same issue but I found a semi-working workaround. Following the instructions for using a remote subsonic server at
http://www.subsonic.org/pages/sonos.jsp I was able to add subsonic as a custom service. I say "semi-working" because while I can browse, results from subsonic don't show up when searching.
Sucks because I finally bought a sonos this week, after reading about using subsonic to get around the native track limit.

I'm actually going to start a new post since I think it's not really the same as the original post.