Prediction exchanges let traders buy and sell contracts on the likelihood of future events. Contracts are structured so the prices can be read as a percent likelihood of an event occurring. Studies of the prediction markets have shown they have an accuracy comparable to that of public opinion polls.
from an aticle titled:

Exchange bets favor Obama for Democratic nomination

Mon Mar 10, 2008 4:24pm EDT

http://www.reuters.com/article/bondsNews/idUSN1046846820080310