I've noticed that frequently I'll see a while box instead of the video when viewing on the web site. This normally happens after I've previously viewed on the web site. For example view on the web site will be fine. Go to a different address. Later come back and try to view again and get the white box. Audio works fine.
When the whilte box shows up I either have to restart the browser for Firefox 3.6, or close the tab in Internet Explorer 8 and open a new one. The reason IE8 doesn't need to be restarted is because each tab runs as it's own process in IE8 so closing the tab "kills" the plugin. While Firefox 3.6 can run some plug-ins in their own process, it doesn't do so for the SlingPlayer plug-in.
It seems to be a graphics issue, but I never saw this before the 22.214.171.124 version of the plugin.