Used recently in the Economist. According to Wikipedia, “shill, also called a plant or a stooge, is a person who publicly helps or gives credibility to a person or organization without disclosing that they have a close relationship with the person or organization”. Also used for hustlers who attempt to entice people into going into a circus or other form of entertainment.
According to the Concise Oxford, the origin is uncertain but probably related to “shillaber”, a circus barker.
Jonathan Green (Crooked Talk: Five Hundred Years of the Language of Crime) considers “shill” as possibly an abbreviation of the Irish “shillelagh”, a cudgel (cudgelling the victim into participation) or earlier oak wood to make cudgels from (http://www.memidex.com/shillelagh, 1670) originally related to the town of Shillelagh in County Wicklow. A person behind the bar with a cudgel to prevent disturbances may also have been known as a shillelagh, which provides some support for the term gliding to become the “shill” for a person at the door.