Do you still need this resolved? I used Dagon Design’s Multi Column Category List plugin and edited it to be used with monthly archives for WP 2.3 and above. Download the following files:
http://www.thehgpproject.com/wp-content/plugins/dd-multi-col-arch.php
and
http://www.thehgpproject.com/wp-content/themes/hgp/dd-multi-col-arch.css
Then make sure you upload them to the appropriate directories (use the URL of the files on my site as a reference).
