Theoretically, perhaps, though the initial uphill battle for traffic is harder, and if you're not careful there's always the chance you're going to get stuck policing and removing your users' content.
It strikes me that there must be huge and quiet money in some forums. One forum in particular I use a lot is Better Bidding, where people discuss how to get cheap rates on hotwire and priceline and share information on what the unnamed mystery hotels are (so far I've had a 100% success rate in finding out the name of a hotwire hotel before booking it). But every time the words "priceline" or "hotwire" show up on the site they're converted into a (presumably) affiliate link, so the owners of the site must be making one metric shirtload from those. If you can find a way to combine user-generated content with affiliate links to stuff that the users were about to buy anyway, that's gotta be some great passive income.
Oh well. If this story did nothing else it at least inspired me to dust off a blog project I've had in the back of my mind for a while (http://projectomniscience.blogspot.com) and experiment with adsense. I doubt it'll be a big money-spinner, but at least now I know how to use adsense (and besides, I got to learn interesting facts about zebras, pants, and governors of North Dakota).
I agree with what you have said, here. There is definitely money in forums, and this is surely why I have seen motorcycle.com buy up lots of forums over the passed couple years.
You describe my biggest hit; people searching for something, finding my site and writing a post or a reply that helps generate more search traffic. I don't have to police much since the sites are very focused (not forums).
