The FTC released additional rules / clarifications for the CAN-SPAM act recently: http://www.ftc.gov/os/2008/05/R411008frn.pdf
The updates include:
1) "An e-mail recipient cannot be required to pay a fee, provide information other than his or her e-mail address and opt-out preferences, or take any steps other than sending a reply e-mail message or visiting a single Internet Web page to opt out of receiving future e-mail from a sender;"
and
2) "A "sender" of commercial e-mail can include an accurately-registered post office box or private mailbox established under United States Postal Service regulations to satisfy the Act's requirement that a commercial e-mail display a "valid physical postal address";"