I've just had a very sick thought. Spammers suffer as, on average, no one at all reads the messages they send out. Worse (for them, not us) many people can stop receiving spam altogether using things like spam assassin. What they need is a method through which people actually want to read their adverts.
Now... There are many, many people, myself included, who have a PGP key but rarely receive encrypted email. This is mostly because there's nothing worth encrypting. Of course, when I do get something which is encrypted the first thing I do is fire up gpg and read it in a rush of excitement.
So, our not so friendly spammers simply need to grab every key from a public key server, encrypt their advert and email it off to all the email addresses in the key id.
But remember kids... Don't do this at home.