Ryan Stewart has a great post breaking down the search engine optimization for Silverlight. Many people still don't realize how search engine friendly Flash is - Google indexes SWF files without any problem. As well, many developers have created applications that allow you to deep link into Flex applications, by changing the URL bar as the application changes. Yahoo Maps is a great example, and there are others.

While Google can index SWF files without too many problems, it will only index the static content that is in the file. If your application goes back to the database to retrieve information, that content won't be indexed by Google. In essence, Google doesn't know how to crawl the SWF file, as it can for dynamic web pages. This is one of the limitations that Flex applications currently have, though some clever developers have developed solutions for it and Google is likely working on it.