In June 17, the Federal Trade Commission issued a press release and formal report related to the CAN SPAM act.

The act required that they look at spam tagging with ADV: or a similar subject line tag. They conclude that tagging would not be effective, mostly because spammers who are breaking the law anyway are unlikely to add tags merely because another law says they have to.

They also looked at 20 state laws and national laws in Europe, Korea, and Japan that mandate tags, none of which have been at all effective. The report is well written and is worth a read by anyone interested in the way that spam policy is made.