Microsoft now have content on TechNet specifically about ForeFront and SharePoint - http://www.microsoft.com/technet/forefront/serversecurity/sharepoint/default.mspx. ForeFront is Microsoft's anti virus product. It includes 8 anti-virus engines to scan content. Up to five of these will be used during any individual scan. This ensures good coverage across a range of attacks. Seven of the engines are from external vendors - AhnLab, Authentium, CA, Kaspersky, Norman, Sophos and VIrusBuster.
There doesn't seem to be many other vendors offering SharePoint 2007 AntiVirus software just yet. McAfee and Symantec still only advertise a SharePoint 2003 product. Hopefully more vendors will step up soon. With SharePoint becoming increasingly popular as the collaboration and file storage platform, anti-virus is going to be a must-have feature.