We had some problems where the visitor got redirected to another site. I have replaced all of the php-files for the forums, checked the templates for anything suspicious, and some other controls to get rid of it. Before that, I managed to get my anti-virus to warn for a redirect malware twice, and none after even after several tries from two different computers. Still, as the redirect wasn't triggered each time, it can mean I was just lucky/unlucky and it is still there.
While I will keep hammering some tests to improve the odds to detect it if is still there, please send me a PM or post in this thread if anyone of you finds it or hear about it from someone else. So far, it has been triggered by following a normal link here. While my anti-virus didn't tell me where it was attempting to redirect, the redirection reported to me was to a download at myfilestore.com.
Thank you The Tatterdemalion King for pointing the malware attack out.