Blacklisting spammers would be quite effective, if it wasn't so easy for them to forge the sender. I've been encouraging friends to sign their messages with S/MIME, in a sort of grassroots effort to work around the primary weakness of the net's email infrastructure (SMTP's blind trust of the From: header) that enables spam.
It's getting easier to obtain a free certificate, and recent versions of Outlook and Mail provide seamless support. Mail even started encrypting mail between Bryce and I as soon as it saw his public key. I don't wear an aluminum hat, but I don't mind knowing it'll take some effort for others to snoop around my messages.
It's gotten on my nerves, though, that my certificate only verifies my personal email address, and does not relate my public or work addresses, or certify my identity. Thawte's Web of Trust will let me do all that, but only after I've been validated by a number of folks. St. Louis isn't exactly on the cutting edge of technology, so this has been a bit of a hassle. If I'm going to preach S/MIME, I should grease the machine by becoming a WOT notary myself, so that's the goal. Let me know if you'd like to attend an "assertion party" and get a jump start.
I think it's inevitable that the WOT shall grow. Critical mass has already been reached, the underlying technology is widely adopted, and the reputation society may depend on it.
For the record, my public key's SHA1 fingerprint is EA D8 F9 3A 2B 50 F8 08 11 B0 37 59 41 E1 57 DF 63 9C 0F 4F