Google scanning for viruses?

Google has added a virus scanning feature to its Gmail Web mail service, complementing the existing virus protection based on blocking certain types of file attachments, such as executables. Google informed users of the new feature on a Web page where the company announces new Gmail features. Now, Gmail will automatically scan all attachments users send and receive, according to a frequently asked questions section devoted specifically to this new functionality. Gmail will attempt to clean or remove viruses from infected attachments so that users can access the attachment's information; otherwise, users will not be able to download the attachment. Gmail will also prevent users from sending messages with infected attachments.
Until now, Google has protected Gmail users by blocking messages that carry attachments commonly associated with virus attacks.

But which anti-virus product are they using ... nobody seems to know ... even my friends on some respected anti-virus forums don't know the answer .. strange?