I made a post on Twitter recently that I think that Apple will give away Mountain Lion to current Lion users. I believe this is the future of OSX updates. I believe for as long as its 10.X it will be free to existing users of the previous iteration. Updating OSX will be a vital component to the future of iCloud. For as long as Apple provides iCloud as a free service they will have to implement future updates of OSX as a free upgrade. This is the strategy they have always taken with iOS. They are integrating iOS and OSX more and more with each release. Within about three years they will be the same operating system.
Some have taken notice that Apple has dropped the “Mac” from OSX. They dropped it because in the future OSX will be on devices that aren’t called a Mac. They’ll always be a Mac, but OSX won’t be just limited it.