Similar to the news we're now hearing about the end of Flash Player support for mobile devices, there are sources around Microsoft that are saying Silverlight 5 will be the last version. There have been no roadmaps or plans for Silverlight 6 released at all, and we're very nearly at the Silverlight 5 release date. That, along with the well-known plans to shift the Silverlight strategy and reorganize the Silverlight team at Microsoft are some strong indications that the Silverlight plugin, like Flash on mobile, could be depricated.
If there is no Silverlight 6 in our future, don't despair. What's happening on Windows Phone and Windows 8 is the birth of something new and powerful, and those with Silverlight skills won't be left in the lurch. It's expected that for some use cases of Silverlight that don't want to migrate to HTML5 or a XAML/.NET stack, if available, will get tools and support from Microsoft in the coming months.