To enhance search engine indexing of the Flash file format (SWF) and uncover information that is currently undiscoverable by search engines.
And this is said on Google Blog
In the past, web designers faced challenges if they chose to develop a site in Flash because the content they included was not indexable by search engines. They needed to make extra effort to ensure that their content was also presented in another way that search engines could find.
But there is no one talking about what is the role of Microsoft Silverlight.
Is Microsoft Silverlight content can be search and index by search engines?
Are these search engines interested searching and indexing Silverlight content?
After searching on web I find that Microsoft has added the technology in Silverlight that help search engines to easily index Silverlight content. What Microsoft officials said on this
Microsoft designed Silverlight from the beginning to be easily accessible by search engines. Because it is simply a ZIP archive, a Silverlight application packaged in a XAP (the Silverlight application-package file extension) file is easily accessible to search engines without a special software development kit (SDK). And because XAML is W3C-compliant XML, any static textual XAML content can be easily parsed by search engines. Furthermore, any metadata embedded in the ZIP file is easily indexed by search engines as well. Silverlight applications also support “deep linking” as they easily consume the URL they were loaded from, and use information on the URL query string to rapidly load and display appropriate data. Finally, the Silverlight DOM itself can be easily inspected to detect all text, links and images that are being visualized by the control.
So does this mean that Silverlight offers customers superior search engine optimization (SEO)? Yes. Not only was Silverlight architected to offer superior searchability, but Silverlight excels at enabling dynamic content published from content management systems to be easily indexed by search engines. By publishing dynamic content to Silverlight via XAML and XHTML mirroring, users are able to dramatically reduce the time it takes to optimize content for search engines.