I don't why i haven't seen this around before, or its not well documented as it would of solved lots of problems in the "IS IE" Js detection scripts from the browser wars. Anyway, in a nutshell IE 5+ excepts HTML conditional comments, just try running the following code:

<!--[if gte IE 5.5]>
\n<SCRIPT LANGUAGE="Javascript">
\nalert("Congratulations. You are running IE5 or later!");
\n</SCRIPT>
\n<P>Thank you for closing the message box.</P>
\n<![endif]-->

Probably not as usefull now, what with browsers becoming more standards compliant, but still, handy to have in the tool bag.