There are plenty of Outlook viruses that infect computer A, mine the address book, and then send out infectious e-mail to parties B, C, D, and E, but pretending to be someone else from the address book, making it much more difficult to trace back to the infected computer and fix the problem. It would be simple for one of these virus writers to substitute spam for infectious e-mail (and probably add in hooks for updating the spam messsage remotely).
I have just received a bounce message for a piece of spam that purports to come from me, and was apparently sent to an invalid address. It is also interesting to note that the entire message text is base64-encoded, which no doubt helps it slip past spam filters.
Needless to say, I am chagrined. For those who care, I have posted the raw text of the bounce message (e-mail addresses changed to protect the innocent).
Meanwhile in related news, Spamotomy looks like a good clearinghouse of information on spam.