I'm one of those people -- original MacBook Air, speculated unsupported by final Mountain Lion release -- who would certainly buy a Mountain Lion upgrade, but I'm a writer with very modest laptop performance needs, and three kids with the usual kid needs, so I won't dash out and buy a new Mac just to run Mountain Lion.
Mountain Lion is scheduled for "summer 2012 release." Summer doesn't end until September. Apple has a tendency to push the seasonally announced release window right down to the wire. I think it's a little early to assume iMessage won't be available, like FaceTime, for a fee in the MacApp Store just on the evidence of the beta expiration language promoting Mountain Lion. Perhaps Apple has already planned to sell it separately. Perhaps they're considering it but haven't decided. Perhaps they've decided against but will respond to demand and offer a MacApp Store version. Perhaps Mountain Lion's supported hardware list will expand for the final release.
Who knows? Wait for an official announcement and then freak out. Or, better yet, email Apple and throw your request up on Twitter, too, and maybe they'll release it on the MacApp Store.
Maybe not. But, seriously, how many of you who are already so enamored of iMessage for OS X don't also own on an OS 5-supported iPhone or iPad, or both? And how far away from your Mac do you keep those devices when you're sitting at your desk? If you already have an iPhone or iPad, iMessage for OS X is a very, very small feature. I'm been pining for a OS X iMessage app and now that I have one I realized with my phone right near me, my first instinct is to reach for it.)
(It's definitely a beta though. Yesterday afternoon it was syncing messages fine across my Air, my iPhone and my iPad. This morning it just quit working in that capacity. I sent an iMessage this morning to my daughter's phone and got a reply, which synced, but it's no longer syncing anything I've sent or received from my phone with other iMessage users.)