Silverlight 1.0 has been released tonight, and it will natively support Windows and Mac, and will officially support Linux via a partnership with Novell and Moonlight, which is part of the Mono Project.
Together with Silverlight 1.0 RTM, also the RTM of Expression Encoder has been released.
A lot of MS blogger already blogged about this announcement, here are a few posts where you can get more info:
You can check out the great Silverlight example that is Halo 3 Combat trailer in HD (720p).
And while I'm on this topic, I just found a great 20 parts tutorial by LiquidBoy: it's about the development of an iTunes clone build in Silverlight 1.1, complete with CoverFlow and themes.