Works like this: the spammer grabs your e-mail address, using it in the from-line of a mass-spam. The subj. lines tend to stay on pretty typically tacky spamtrack: drugs, enhancements, etc.
That from-line, though, looks legit and, more seriously, looks legit to the filters standing watch over the recipients' mailboxes.
And when the spam hits a dead address, guess who the mail bounces back to?
You -- that's who.
While not an immense problem at the moment -- unless of course you're the one being bouncebacked or backscattered, the effectiveness of the tactic (messages that look more legitimate to filters are likely to look more legitimate to users, too, resulting in higher open/read rates) all but insures that it will become more common.
Needless to say, the security/filter industry is gearing up to do backscatter-battle.