Silverlight 2 supports local data caching and isolated local storage, which is one reason to consider it for use as a desktop RIA, although like Google Gears it runs inside a browser. There's also another scenario to consider: With care, you can write a Silverlight Web application that can also be recompiled to be distributed as a WPF desktop application, and the WPF application would stand alone but be able to connect to the Internet as needed. Note that it's much easier to write a Silverlight application and recompile it for WPF than it is to take a WPF application and rewrite the parts that aren't supported by Silverlight.
Silverlight 2 runs as a runtime on Windows XP, Windows Vista and Mac OS X on Intel-based Macs. Current versions of IE, Firefox and Safari are officially supported, with "others pending customer feedback"; I have also used Silverlight 2 successfully with current versions of Google Chrome and Opera. The Microsoft development and design tools require Windows; an Eclipse plug-in for Silverlight currently requires Windows but will eventually support other operating systems. The Linux implementation of Silverlight 2 is called Moonlight 2 and is currently in an alpha state.
Silverlight has copious documentation and very good tool support. Developers typically work on Silverlight applications using Visual Studio 2008 on Windows; designers typically work on the same project using Expression Blend 2, also on Windows. Both of these tools are very good for their target audiences, but not for their secondary audiences. That is, Visual Studio 2008 is great for coding and XAML text editing and can preview XAML pages, but can't do visual XAML page design; Expression Blend 2 is great for visual XAML page design but doesn't support coding. The future Visual Studio 2010 product, now in preview, will combine both sets of capabilities.