Ads for a free iPad that are popups are spam and spyware. If they are not, they will lead you somewhere that makes you fill out a million surveys. Therefore, 99% of the time they are not valid contests. However, I know of radiostations that hold contests for iPads that you can win.
Some legit contests where an Ipad is a prize may be out there but as for those pop up ads that ask you to fill out a survey or sign up for something to win one, those are all scams. You shouldn't do these they could place spy ware or worse on your computer.