With AdSense, Google does not allow you to encourage people to click on your contextual ads. But this rule does not apply with the referral products that AdSense allows you to market.
A WebmasterWorld thread asks this question and AdSenseAdvisor comes in to clarify;
I don't know if this is the page you already found, but you might want to check out the referrals tips at:
"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
Some may forget this.
Forum discussion at WebmasterWorld.