"Unlike with AdSense for content ads, we encourage you to endorse referral products by calling attention to the button or text link. If you believe in the quality of the product that you're referring, feel free to let your users know! Of course, in line with our policies, you may not click your own ads nor encourage conversions for deceitful reasons."
Hope that helps! -ASA
[edited by: AdSenseAdvisor at 1:40 am (utc) on Dec. 19, 2006]