I still don’t understand what’s wrong with Jabber. The online/offline/unavailable stuff is a bit different from the always-on state of SMS, but just stay always connected and it’s pretty much the same thing (and even more informative with “away” indicating that you’ll receive the message but might not read it on time).
It’s open, it’s free, it’s available on pretty much every platform that ever existed. And with GTalk based on it it has a huge install base as well.