A eye tracking study jointly carried out by the marketing firms Enquiro and Did-it, along with the eye tracking firm Eyetools, on sample Google search results has shown that (perhaps unsurprisingly) your search ranking matters a lot if you're looking for traffic.
The results aren't startling, although I was surprised to see so little attention was paid to the AdSense adverts. However I realised that unless I'm trying to find somewhere to buy something really odd and don't even have a clue where to start looking, I rarely even glance at them myself. It is interesting to see that unless a company ends up as the top advert in the AdSense link column they're very unlikely to get any traffic.
However what was really interesting is that this study confirmed what I always figured, despite people's claims to read the search results and choose the most appropriate link, they rarely look past the top one or two. As Gord Hotchkiss said in the release "We see a marked difference in how people say they search and what they actually do. Previous research had indicated that people were considered searchers and spent some time before choosing a link".