Gary Price at the SEW Blog reports that Google Launches Anti-Virus Scanning to Gmail.
This addition seems to have been added yesterday according to the FAQs page.
How does anti-virus scanning work?
Each time you send and receive attachments, Gmail automatically scans them for viruses. If a virus is found in an attachment you've received, our system will attempt to remove it, or clean the file, so you can still access the information it contains. If the virus can't be removed from the file, you won't be able to download it.
If a virus is found in an attachment you're trying to send, you won't be able to send the message until you remove the attachment.
I posted a thread on this topic at Search Engine Watch Forums.