I believe in RIAs because developing an HTML/AJAX application is just painful.  And I believe that Silverlight provides an excellent RIA solution.  For two great examples of Silverlight in action look at:

http://windows4all.com/

and,

http://www.worldwidetelescope.org/webclient/

These both look and feel like desktop applications but are running safely in your friendly browser.

Be aware that CodePlex has a project for windows4all.com (http://windows4all.codeplex.com/) but all of the source code is not provided.  Just a Visual Studio shell application.  It would be great to sneak a peek at the code under the covers.