i personally got 35% off of a new eee pc as i posted on the forum here.
as the poster stated, you search through live.com, you can alter your search on ebay once directed there from live.com but after clicking buy it now it will confirm if and how much money you'll get back (before finalization). then after purchase ebay sends you a message with a link to signup on microsofts site, then it will show the money cash back pending, it will post to your paypal within 60 days... just make sure on the microsoft site you input your correct paypal email...