Our svn server sends out emails. We send emails from svn to our homgrown build system as event dispatches, which treats incoming emails as a message queue, filters out relevant events based on svn path in the message body, and start new builds accordingly. We find this approach to be efficient (event is dispatched immediately), less risky (our svn server can avoid potential hangs because because it is not opening tcp sockets via http requests), and low cost with a proven technology (email and smtp).