Google recently held a webinar for AdSense publishers. They gave out some adsense optimization tips that they have gathered over the past few years. The transcript is pretty lengthy, so here's the summary:

Ad Location - "the middle, above the fold location perform best.

I just logged into my Google AdSense account, and noticed that they updated the terms, and also are introducing AdLinks.

Ad Links are a new form of text advertising that we're offering to our AdSense publishers.