I was able to get code assist up and running in an HTML file in Visual Studio 2010 (Professional SP1) by copy/pasting one of our published samples and adding an identical script reference tag to the one you posted above.Something I noticed though is that, within the HTML file, the intellisense only appears within script blocks explicitly declared as javascript<script type="text/javascript">
//intellisense will appear here, but not outside the block...
</script>
Does it make any difference if you use a full http path to reference the script instead of a relative path?