I learned about it here: http://www.techcrunch.com/2007/04/30/silverlight-the-web-just-got-richer/ - a techcrunch article. That artice does a VERY nice job explaining the site, so stop there first.
Then RUSH over to http://silverlight.net/ and click the Watch It button on the right side to watch movie that demonstrtes some of the power of this tool.
Make special note of his statements at the end f that video. You can get the code to embed this into a blog! Talk about power!!
Now, hurry on over there to see this stuff before it gets blocked!