Many businesses are relying on Microsoft Silverlight and until few month ago, Microsoft did not have a clear support policy for Silverlight, well now we have it.
Silverlight is unique compared to other products in the sense it is serviced through new version releases (so new releases not only add functionality but also fix issues in previous versions). This allows Silverlight applications to continue to run correctly regardless of the runtime version.
The official languages is provided here: "Microsoft will continue to support all versions of Silverlight by shipping updates to the latest version of both Silverlight runtime and Silverlight SDK. Updates and new versions of the Silverlight runtime are backward compatible with web applications built in previous versions of Silverlight and will include the latest security enhancements, performance improvements, and product fixes. Previous versions that are still under support will be updated in the event the Microsoft Security Response Center (MSRC) determines vulnerability has been found in Silverlight."
You can read the full details here: http://support.microsoft.com/gp/lifean45