It is not an uncommon occurence and depends on the links structure and the page delivered on the other end of the link.
With some sponsor programs this happens regularly... I had a ************* link as a top 10 at Google for some time..
Programs like
OutbackCash have links coddes that are often picked up by SE's...
http://www.google.com/search?num=50&...=1&q=chubbybbw
DD