I don't know why this hasn't been reported. But vBulletin installations running VBSEO should be updated to the latest version. There is an exploit going around. More information at VBSEO.com.
A forum I frequent triggered an antivirus popup warning of a trojan download and then a few weeks later it actually started redirecting referrals from Google. I did a site: search of the forum for the word viagra and the Google cache shows the alternate web page the forum is redirecting to. Nasty stuff.