Has this started working in Mountain Lion yet? I'm getting mixed reports with iOS 6 beta 3 and I haven't been able to get it to work on ML but it works perfectly on iOS.
I don't have the whole "unified" aspect they showed in the demo (I think the earlier post about needing iOS6 is right) but to enable your iMesssage account in the new Messages app, go under preferences and "Accounts". For some reason it's listed, but you have to login with your password to enable it. At least that's how it was for me. Hope that helps.