OK, here's the CodeMirror magic. You're right; it does sniff the content. The code below loads Perl and HTML/XML syntax highlighting, including support for embedded CSS and Javascript. These seem to be the most commonly used syntaxes on PerlMonks.

'Tis awesome. Think I'm going to keep this in my Free Nodelet.

<link rel="stylesheet" href="http://buzzword.org.uk/2012/codemirror-2.
+36/lib/codemirror.css" type="text/css" media="all" />
<script src="http://code.jquery.com/jquery-1.8.3.min.js" type="text/ja
+vascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/lib/codemirro
+r.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/perl/per
+l.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/xml/xml.
+js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/css/css.
+js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/javascri
+pt/javascript.js" type="text/javascript"></script>
<script src="http://buzzword.org.uk/2012/codemirror-2.36/mode/htmlmixe
+d/htmlmixed.js" type="text/javascript"></script>
<script type="text/javascript">
$(function() {
    $('p.code').each(function (i, c) {
        var $code = $(c).find('tt.codetext').text().replace(/\u00a0/g,
+' ');
        var $dl   = $(c).find('span.embed-code-dl');
        
        var $pre = $(
            '<textarea class="CodeMirror">'
            + $('<div/>').text($code).html().replace(/\s+$/,'')
            + '</textarea>'
        );
        if ($dl.html()) $(c).after(
            '<div style="text-align:right"><small>'
            + $dl.html()
            + "</small></div>"
        );
        $(c).replaceWith($pre);
        
        CodeMirror.fromTextArea($pre.get(0), {
            lineNumbers: true,
            readOnly: true,
            matchBrackets: true,
            lineWrapping: true
        });
    });
});
</script>
[download]

Note that you also need the "Auto Code Wrapping" setting turned on in Display Settings for this script to work.

Updated 2012-08-12: script now replaces non-breaking spaces with normal spaces; added above note about auto code wrapping setting.