Microsoft has finally released their new version of Silverlight, Silverlight 3. All the dev tools are now available here: http://silverlight.net/GetStarted/ .
However, if you want to develop on a Mac, you will need to use something like Eclipse4SL, which, when we tested it a few months back, was stil la bit buggy. However, it's worth trying again.
One of the most interesting new features of Silverlight 3 is support for 3D planes and BitMap caching. It still seems like they are still playing catch-up to Flash though. If they really wanted to make splash, why not leap-frog Adobe instaed of looking like an also-ran?.