Not a shocking announcement. Apple has an agenda to push, namely getting people on to OS X 10.8 (Mountain Lion) and moving forward. I'm not upset at their decision, however, I do wish they would continue to support Lion. This may push my hand to update my unsupported Mac Mini to Mountain Lion (here's a guide), but I am probably just too lazy to deal with hacks.
On the other hand, I would gladly trade support for Messages on Lion for slightly more stable performance on Mountain Lion. I would probably trade a kidney now that I think of it. So, Apple, there's a kidney and a hostage-style five star rating for your app on the table.
Your move Apple, your move.