Google added Sitelinks to AdWords back in November of last year, here is how they look:
So what happens to your traffic when you activate the AdWords Sitelinks when you already have the number one organic listing in Google?
The SEOmoz post has one case study. Here is a table he shared with the impact:
|Visits||Without Sitelinks||With Sitelinks||Change||Change %|
As you can see, adding in the AdWords Sitelinks pretty much didn't end up sending that much more traffic to the site with the already good organic ranking. But if you do not rank well organically, then it does take a away a lot from that top listing and boost your traffic significantly.
There is more graphs and data in the SEOmoz post.