The service location protocol, SLP, discusses how to find services which are in your administrative domain, it's a lot less good for searching wide area, especially when you don't want to find the service closest to you. Jonathan Rosenberg (Bell Labs/Columbia University) presented a summary of draft-ietf-svrloc-wasv-00 which discusses an extenstion to the SLP to use multicast service advertisments, in a similar manner to SAP.
Two generic concerns were raised: lack of multicast deployment, and lack of a reliable multicast infrastructure. Both are clearly valid concerns at present, but in the medium term, it is expected that these problems will be solved.