Word from Gmail team at Google this afternoon that they've just launches anti-virus scanning to the service for all incoming and outgoing attachments. More about how it works here. No word if they are using they're own virus scanning software or licensing it from a third party.
Postscript: Via a Google spokesperson, we've learned that the Gmail anti-virus scanning software is coming from a third party. However, Google would not name the company.
Want to discuss this new Gmail service? Check out this thread in the Sew Forums.