Google has expanded and clarified their Webmaster Guidelines. Everyone that builds any type of site should read this in detail, but here are the parts that specifically apply to affiliate marketing.

“Avoid “doorway” pages created just for search engines, or other “cookie cutter” approaches such as affiliate programs with little or no original content.”

“If your site participates in an affiliate program, make sure that your site adds value. Provide unique and relevant content that gives users a reason to visit your site first.”

Thin affiliate sites: “These sites collect pay-per-click (PPC) revenue by sending visitors to the sites of affiliate programs, while providing little or no value-added content or service to the user. These sites usually have no original content and may be cookie-cutter sites or templates with no unique content.”

“There is no problem in being an affiliate as long as you create some added value for your users and produce valuable content that gives a user a reason to visit your site. For example, you could create product reviews, ratings, and product comparisons.”

Search Engine Roundtable has more.