Looks like "SocksPort 0" doesn't actually disable the socksport.
I tried starting Tor with SocksPort 0, and I got:
Aug 09 13:38:02.418 [notice] Opening Socks listener on 127.0.0.1:0
Aug 09 13:38:02.418 [notice] Socks listener listening on port 57454.
Oops. That sure isn't what we had in mind. It's probably related to the client listener refactoring that I did on my prop171 branch.