Yep, that's a new ad placement. UGO has requested more impressions of the 300x250 ad, so I've snuck one in on the forum view and thread view pages. They won't show up on any of the other forum pages (profile pages, WYSIWYG pages, etc.). If having the forum jump dropdown on the left side hinders any navigation, please let me know.
You'll also see the skyscraper ad on the left navigation bar getting slightly bigger (160 pixels in width instead of 120 pixels in width) within the next week or two. These are just new advertising changes in response to publisher requests for 2007. The good news is that, ideally, it should provide us with a little more revenue to pay bills with (and a few more feet of money to roll around in late at night

).