Today a minor update to Silverlight 4 was released. This update won’t effect all users, but if you are a media developer you will want to get this update. For the specifics read KB982926.
Developers can update their runtime by downloading and installing the updated Silverlight developer runtime for Windows or Mac. Any new Silverlight 4 applications will target this updated version. For existing applications, if your users must have the newest version, update your <object> tag by setting the minRuntimeVersion to 4.0.50524.0.