You can try out the beta for the new Messages app that’ll be in OS X Mountain Lion right now, but once the release rolls around, you’ll have to install Mountain Lion to use the full version.
That’s totally reasonable. It just stinks if you’re one of the unfortunate few who can use the beta on OS X Lion, but will get left behind because your Mac isn’t compatible with Mountain Lion. [Consomac via Macrumors via Cult of Mac]