I was just wondering what ad placements work the best for everyone. For me, a left sidebar skyscraper ad used to perform the best, but I'm running a right sky at the moment. A leaderboard beneath the first post of forum threads also did very well.

People are more and more adsense-blind these days. Large rectangles are by far the most effective for me. However, they're a bit obtrusive. Strategically placed towers are also very effective.

The source of traffic also plays a significant role in CTR. People who find your site through your Adwords ads are more likely to click on the contextual ads on your site.

I have definitely experienced that - Whenever I run a large AdWords campaign, my AdSense ads just always seem to do much better.