A minor rant:
For a while, I got loads of virus-infected e-mails from one particular ISP. I looked back and found I'd received a newsletter type e-mail from one particular person in which the "To:" addresses weren't hidden, and a few of the folks I received the virus from were on his message. I figure somebody on his mailing list was infected. The virus harvested addresses from the header of his message and sent them on. Then somebody else got infected, and yada, yada, yada.

Anyhow, anyone who sends out mass mailings of ANY sort ought to hide the "To:" addresses. They might not be infected, but it's possible someone they're sending the e-mail to IS. (Not only with viruses but spammer spyware!)