As this forum runs vBulletin, I know that the forum I ran for a magazine (I've now left the company) was hacked.
My understanding is that it's rogue code put in an ad slot. And it's clever enough to divert the first time, and then (presumably using cookies or the IP address) working the next. So, most people would be redirected to a dodgy site (and invited to buy some scareware or something) but not the second time - making it incredibly hard for someone to spot. Especially when so many ad slots are dynamic, and based on the content of the page or other factors (cookies on machine showing what sites a visitor has been to recently).
It needs to be fixed, however, as sooner or later, Google will start flagging it on search results as a dodgy site - and then the traffic will fall hugely. It may even result in pages being removed entirely from Google (although you can apply to be rescanned once you can confirm the problem has been fixed).
I clicked the link as detailed at the start and did get a redirect, although Opera then picked up on it as being dodgy so I never proceeded. Upon clicking again later, it worked fine. Let me stress again; the admins on here must take it seriously! The page linked may have subject matter that is getting certain ads to appear, which are infected. If it's Google AdSense, there may well be a case of reporting the code to Google to fix. For the record, we also used AdSense.
Finally, in case anyone is worried - I think that the hack is simply to get you to go to another site to buy something, rather than something to infect your machine.