One theory for the change is that Google is following Bing’s lead. Running a search on Bing will give you quite similar looking results - sponsored search ads appear fairly close to the organic search results.

Furthermore, according to a study that User Centric performed earlier this summer,

Sponsored links on the right attracted more attention on Bing (~42% of participants per search) than they did on Google (~25% of participants per search).

The big question is will this new ad location drive more attention and more importantly, more qualified clicks to advertisers’ web sites? Only time will tell. I certainly will be monitoring advertising performance to see if ad location does indeed have an impact.