I believe the ad placement will also depend on your website and its layout.
While some of my blogs perform better with ads placed on the right side, others perform better with it placed on the left.
I always place my content above the fold (though sometimes I do put a large banner above the content) - and I also work to make sure the ads do not interfere with the visitors' reading pleasure.
Another trick I use (mainly on my more personal blogs which have a lot of repeat visitors who may be ad blind) is to show them ads based on where they come from.
If they're referred from Google, Yahoo!, MSN, or other search engines - they will receive the Google AdSense ads and affiliate links, if they come from another website, they will most likely receive some on topic affiliate links - if they come from a bookmark or direct address, they only get my Google AdSense referral button.
I like to experiment
