Just drop it in your page before the </body> tag:

    <script language="javascript" type="text/javascript">
        var prm = Sys.WebForms.PageRequestManager.getInstance();

        prm.add_initializeRequest(InitializeRequest);
        prm.add_endRequest(EndRequest);

        var oldClass = "";
       
        function InitializeRequest(sender, args) {
            var postBackElement = $get(args._postBackElement.id);
            if (postBackElement != null) {
                postBackElement.disabled = true;
                oldClass = postBackElement.className;
                postBackElement.className = postBackElement.className + " disabled";
            }
        }
       
        function EndRequest(sender, args) {
            var sourceElement = $get(sender._postBackSettings.sourceElement.id);
            if (sourceElement != null) {
                sourceElement.disabled = false;
                sourceElement.className = oldClass;
            }
            oldClass = "";
        }
    </script>

And add this to your CSS (you may differentiate this among various tags like img.disabled for imagebuttons or input.disabled for textboxes etc.):

.disabled
{
    FILTER: alpha(opacity=70);
    BACKGROUND-COLOR: gray;
    opacity: 0.7
}

Just be carefull if you change the class of the element in question (that originated the ajax postback) to register a client script block to set oldClass to the new value as well, otherways it would be overwritten.

At one my clients occured the problem of all videos being played 3 times 3 times 3 times instead of just one.

After some investigation, it turned out, that for each request coming for a WMV file instead of an ASX file is returned. For example, for a request for http://stream/stream.wmv the following asx was generated:

<asx version="3.0">
  <title>Stream</title>
<entry>
    <title>www.stream.hu</title>
    <ref href="http://stream3/stream.wmv" />
    <author>Stream</author>
    <copyright>(c) 2008 stream</copyright>
  </entry><entry>
    <title>www.stream.hu</title>
    <ref href="http://stream1/stream.wmv" />
    <author>Stream</author>
    <copyright>(c) 2008 stream</copyright>
  </entry><entry>
    <title>www.stream.hu</title>
    <ref href="http://stream2/stream.wmv" />
    <author>Stream</author>
    <copyright>(c) 2008 stream</copyright>
  </entry>
</asx>

If you read the specification for the ASX file format, this should/may be rewritten as:

<ASX VERSION="3.0">
<TITLE>Stream</TITLE>
   <ENTRY>
   <TITLE>www.stream.hu</TITLE>
      <REF HREF="http://stream3/stream.wmv" />
      <REF HREF="http://stream1/stream.wmv" />
      <REF HREF="http://stream2/stream.wmv" />
      <AUTHOR>Stream</AUTHOR>
      <COPYRIGHT>(c) 2008 stream</COPYRIGHT>
   </ENTRY>
</ASX>

This would instead of playing the three items after each other just choose the first one, that responds in a timely fashion from the list. The day is saved, the roundrobin load balancing still works as it should, and nothing plays three times three times three times.