Also - from the current affiliate guidelines page on Google.
We'll only display one ad per search query for advertisers sharing the same top-level domain in the display URL. This means that if you're an affiliate advertiser, your ad may not show for a query because another affiliate or the website that runs the affiliate program also has ads using the same (or a similar) domain in the display URL. Also, your site should not mirror (be similar or nearly identical in appearance to) your parent company's or another advertiser's site.
One of my sites has some affiliate products for sale that are related to the topic of the site. It's not a pure affiliate site by any means and I write my own copy for the items, and use my own templates, and receive extremely favorable results. I used the affiliate copy on one item when in a hurry a week ago with plans on updating it later, the quality score couldn't be lower. 10.00 minimum. I updated the page and the quality rating improved drasticly.
I'd say thats pretty impressive of the algo to sniff out on a page per page basis, don't you?
[edited by: JS_Harris at 6:34 pm (utc) on Sep. 20, 2007]