It works for me. You don't need to include the minor version number, "5.1" is enough. I can't check with 5.0, but if I set it to "5.2", I get a "This page requires a more recent version of Silverlight" dialog. The version number in the link doesn't do anything. The linked-to page always downloads the latest version (for obvious reasons).
<param name="minRuntimeVersion" value="5.1"/>