According to this ZDNet article, Google will be adding anti-virus capabilities to it’s GMail service. If you have a GMail account, your “What’s New” page may already reflect the following text.

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.

It will be interesting to see if this additional scanning slows the service down at all.