Common challenges with Silverlight and Search Engine Optimization (SEO).
Crawlers cannot examine \ index XAP files and only see the object tag. Microsoft may be researching how to do this for Bing. This is just my assumption, I have not seen anywhere that they have stated they would do this.
Even if the crawler can open the XAP file, it would not see the dynamic content displayed to the client.
Not everyone has Silverlight installed and there are a few browsers do not support the Silverlight plug-in
Meta tags and title do not change in the html output as you navigate through the Silverlight application
Links to specific pages inside the Silverlight application usually do not exist outside of the application
Silverlight does not produce HTML content natively