Positioning of Adsense ads is a tricky one. Adsense ads need to be prominently seen and on clickable locations on the page. But they should not be located where users get confused that its part of that sites content. Google has had several recent directives on the subject:

I was forwarded an email from Adsense asking us to move the images a client had placed close to Adsense ads. The ads could have been thought to be a description of the images. Until now this had been a common method of promoting Adsense ads.

But Google’s policies regarding positioning of Adsense ads near navigation created considerable uncertainty. Their second post, quoted above has helped alleviate some of that uncertainty.