You will find this very interesting:
http://www.ftc.gov/opa/2009/10/endortest.shtm
Changes Affect Testimonial Advertisements, Bloggers, Celebrity Endorsements
The Federal Trade Commission today announced that it has approved final revisions to the guidance it gives to advertisers on how to keep their endorsement and testimonial ads in line with the FTC Act.
The rules become effective 12-01-2009