Someone recently asked about this on the dev list:
http://groups.google.com/group/omeka-dev/browse_thread/thread/ada439144a24deb7
You'll need to open the 'item-metadata.php', in application/view/scripts/items, that governs the display of item metadata.
custom_show_item_metadata(), which uses show_item_metadata(), uses this template file for formatting.
If you want to remove the 'Dublin Core' heading, you could just copy the 'item-metadata.php' file to your own theme (in theme-name/items/), and remove the line '<h2><?php echo html_escape(__($setName)); ?></h2>'. this will remove the Dublin Core heading, and all subsequent headings for element sets.
If you only want to remove it for Dublin Core, you could just do a check on $setName, something like this:
<?php if ($setName != 'Dublin Core'): ?>
<h2><?php echo html_escape(__($setName)); ?></h2>
<?php endif; ?>
While you're at it, you can of course modify any of the other HTML for element sets, element names, and values as you'd like in this template file.