I think sometimes about extending xml-rpc for event support. IMHO it is possible, but not simple, however interesting :).
Event interface need multi-threading support on client side - client must use "event listener thread". Of course it should be embedded in library, because using event should be extremely comfortable to use :)
Probably, every subscribed (by client) event interface should use separate thread (only on client side/server side/both ?), but it still need analyse and design.
Other trouble is not pure application of http. How you note http is request-response protocol. However (IMHO) http push is possible by multipart/x-mixed-replace content. If you are interesting for this way, we can try to design this solution.
Of course instead of push is possible, to use active client, that ask server for events, but it is not scalable, and I'm not interesting for this way.
-- Best Regards Marek Zakowicz
