Here's where I found it in workshop/locallib.php:
echo "<fieldset class=\"invisiblefieldset\">";
echo "<div class=\"invisiblefieldset\">";
That worked for me on 1.9; hope it helps others. But as per http://tracker.moodle.org/browse/MDL-11119 I personally would like to see the class="invisiblefieldset" simply go away on this form, as I don't see a real need for it here and switching to DIV affects XTHML semantics--but then again this is my first look at it, so I may be missing a hidden purpose.