Microsoft and others published the WS-Discovery specification this week.  This provides an ad hoc mechanism for services to advertise themselves and for service consumers to track them down.  If you throw UDDI into the picture, it provides a mechanism where services could potentially add themselves to a UDDI directory or at least request to be added to a UDDI directory.  If you take it to the next step you can see how a DNS-like service directory system could be created on top of this protocol.

Check out Omri and Jorgen's posts on WS-Discovery too.