You mean it wasn't already "released"? Nope. Even though we've all been using it up the wazoo, it had not actually been released. There was some interesting agreements in play that made it available to us while dev was still working on it.
So, link below is to my friend Tim Heuer's blog. He is on the Silverlight team at MS and always writes his blog in an easy to follow way. He's the go-to man with questions.