Silverlight is both client- and server-agnostic. There’s no difference between the Macintosh and PC runtimes; you don’t need any Microsoft software on the server if you don’t want to – you can deliver a great Silverlight experience from an Apache / Linux server to a Mac OS 10.4 client.
So the technology is both client and server agnostic. and it runs on mac. Wow. And the Microsoft Silverlight Fact Sheet says
Silverlight is delivered to end users through a small one-time installation, offering consistent experiences to both Macintosh and Windows users on a variety of browsers including Internet Explorer®, Firefox and Safari. Silverlight will be available to customers on April 30, 2007.
So the plugin is installed in more or less the same way as flash, and from day one support is offered for Windows and Mac users – even for Safari. I can’t find a clear indication of whether XAML itself will utlimately be an open standard or not, but based on Adobe retaining copyright on Flash (according to their current EULA for the player) I would expect it not to be totally open. Having said that xaml content can be embedded in html documents as clear text xml. That’s something you could edit with Notepad. The next few year in Web are going to be exciting to be a part of