* for messaging why would you not use xmpp and/or smtp?
As for using http -- I'd say http makes good sense for getting, sending and receiving documents -- like text and images. It makes it fairly easy to implement a REST-like architecture.
As for things that have been tried before, we have ping backs -- that IMNHO never really worked. And there's Diaspora that have yet to come up with a stable protocol -- and have an implementation that is pretty badly broken.
It's also a good illustration of going the "full http" route: publishing becomes easy; interaction (server to server) becomes hard if you want to have any kind of security in place.