I've recently (last 2 weeks) had a couple of emails returned 'rejected' because I had sent them infected with Klez.In fact, I hadn't sent them at all and my address (in this case used for a specific web page and so not my normal one) had been 'spoofed' from someone else's address book. If those recipients hadn't had a virus checker they'd be cussin' me without reason.
A pain, but as long as there's people out there without virus checkers and as long as there's people who wish to proliferate viruses etc. it's something we'll have to work around.
Hey ho! Nick