Ok, take that back.
Apple probably do have to charge for Mountain Lion because of the Sarbanes-Oxley Act, which basically means you can’t add additional functionality to something you’ve already sold. They get around this with iOS devices because Apple account for them over 2 years in a subscription model – you get free updates because Apple treat it as if you’re still paying for it! The Mac isn’t accounted for like this so thus they can’t do free updates.
Rats. Maybe it’ll just be a token cost?
So, Mountain Lion is coming this summer. Woo! I personally like how they’re unifying different ideas and sorting out weird inconsistencies between iOS and OSX (like making Notes separate rather than part of Mail, creating a separate Reminders app, and renaming iCal to just Calendar). Messages also is pretty handy, and now available in beta form. The way it was announced was also interesting: no press event, but instead one-to-one presentations with key Apple writers.
My prediction is that it will be a free update to all users of Lion. The benefits of having everyone on the latest OS release far outweigh any revenue they may get from the update. iOS updates are free for this very reason.