Hey Ungoliant. We're attempting to monetize the forum content with an ad. As you can imagine, there's hardly any advertising revenue on the subject of spiders and we're experimenting with other methods that are not too intrusive.
Ads will show up in the right side of the first post of a thread. After we accumulate some data over the next 7-10 days, we may abandon this placement. Newer threads will have a delay before ads are shown as Google needs to crawl/recognize that content before delivering an ad.
I use Firefox with Adblock Plus, so I don't normally see ads. I loaded this thread in Internet Explorer, and what a difference! There is a rather obtrusive Flash ad urging me to "Become a YouTube Partner" occupying the right half of the first post (as you described). This is causing the text to be wrapped at the same place in every reply, even though there are no ads in the replies.
I don't know if you're still considering where to place the ads, but they would be a little less obnoxious if they weren't embedded in the posts themselves. I understand your desire to monetize, and I realize that advertisers often want their ads to be as "in your face" as possible, but if ads are too obtrusive, users end up blocking them or leaving altogether. (If I were an Internet Explorer user, I doubt that I would have stuck around.)
It is, however, kind of amusing to see what ads Google thinks are relevant to my posts. Most of them are about pest control (and tools to kill spiders), which runs counter to the spider-friendly message that the site often tries to convey. Then there are some odd ads like cat sweaters (yes, cat sweaters), local real estate, and a TV show called Shark Wranglers.
I will remove the ads from this position in the forum this week. They are not performing at all =).