This new report from the FTC, False Claims in Spam (16 page pdf), states that is the first extensive review of deceptive and unfair practices that appear in unsolicited commercial e-mail (UCE). The study analyzed 1,000 spam e-mails drawn from a pool of 11,000 such messages, and concluded that at least one form of deception was evident in 66% of the aggregate.
See also these two related agency reports, Spam Email: Harvesting Your Email Address and Remove Me Surf. In addition, in related news, Web Sites Shut Down in Spam Fight.