Everything you purchased, assuming its not the 2 movie companies they don't have wrapped up yet (sorry cannot remember) the way it works is anything you have purchased from iTunes whether TV Shows or Movies you can re-download for free on ANY iOS device/iTunes. So lets say you bought the Dark Knight at 720p a year ago, well now you can delete that copy from your iTunes library and download a fresh 1080p copy instead. No penalty for re-downloading...
This is just a stepping stone before iTunes Movie Match hits, probably late this year or maybe WWDC *Crossing Fingers*