Back in February Apple shocked pretty much everybody by announcing OS X 10.8 Mountain Lion without warning. At the time, they released a developer preview of Mountain Lion, as well as a public beta of one of Mountain Lion's tentpole features: Messages. Just like Messages on iOS, Messages for OS X allows users to sync their iMessage conversations across devices. With Mountain Lion, you can start a conversation with a fellow iOS or Mac user on your iPhone, and finish it on your Mac. Despite a few early bugs, it's quite an amazing feature. Happily (and as you might expect) the public beta was compatible with OS X 10.7 Lion, so if you wanted to give Messages a shot but couldn't access the developer preview, you were in luck.
Unfortunately, all was not well in paradise. The day after Apple's announcement, it became apparent that Messages beta would only work on Lion for as long as it took for Mountain Lion to become available. Once Mountain Lion releases, Messages on Lion will stop working.
Now, with Mountain Lion scheduled to release sometime next month, Apple has decided that they've already got all the beta testers they needed. Sometime yesterday - it's not quite clear when - Apple pulled the Messages beta from their site. So if you don't have Messages on your Lion Mac, you never will. What's more, if you already have Messages on your Lion Mac but can't upgrade to Mountain Lion (case in point, my MacBook is too old for Mountain Lion), your days with Messages are numbered.
It's not entirely clear why Apple chose to make Messages Mountain Lion exclusive. After all, the fact that the beta version worked with Lion suggests that the full version would as well. The obvious answer, of course, is that they want to encourage Lion users to upgrade to Mountain Lion. After all, most of the Macs that were compatible with Lion are also compatible with Mountain Lion, and the upgrade costs only $20. Unfortunately, those who can't upgrade (or just don't want to, for whatever reason) are left in the lurch.