Read about the new rules that are coming Dec 1 2009 from the FTC http://redfoxseo.com/2009/10/ftc-new-advertising-guideleines-and-what-it-means-to-affiliates/
There is already a detailed thread about the FTC issue and it even includes a FREE Disclosure Policy Generator you can use. New FTC Rules, Fines to $11,000 - Affiliates and Bloggers Must Read