Download Messages Beta Now
Mountain Lion may be a few months out still, but you can try out the messages app right now. Only question that remains is: will iChat be rolled into Messages? It would make some sense, but I doubt Apple would like to dirty its iOS pool with messages from those other companies.
Update: After the install and a reboot, my question was answered. iMessage is a new protocol in the new version of iChat, now called messages. So still no cross posting between iMessage and other protocols, but at least you have the one app to rule them all.
Not that iChat is the best multi protocol messaging client…far from it….but it is the only one that does iMessage.