Agent frameworks have been around forever, as a result most carry lots of pre-web baggage.
In particular, the content of a query message would be SPARQL, the content of an inform an RDF graph. The typical agent would have the capability of serving HTTP as well as acting as a client.
The mime type of the HTTP message could supply the basic message type, although additional performatives (such as subscribe) would presumably require the use of other mechanisms (HTTP header extensions?)
Alongside the use of inter-agent message passing, agents could share an RdfStoreWhiteboard
See also: JADE, the Java Agent DEvelopment Framework makes it easy to build mobile agents. @@TODO add overview
I've started coding on this, nearly got to Hello World!, will make source public as soon as I get a minute to tweak subversion settings - DannyAyers
Ludger van Elst, Andreas Abecker, Ansgar Bernardi, Andreas Lauer, Heiko Maus, Sven Schwarz: An Agent-based Framework for Distributed Organizational Memories. In Coordination and Agent Technology in Value Networks, Multikonferenz Wirtschaftsinformatik (MKWI 2004), 9.-11.3.2004, Essen. - LeoS works with some of these guys.