Along with the new interface and new software update for the Apple TV, Apple has told us that it has another little bonus for iTunes users. With the update to 1080p support on the 3rd-generation Apple TV, video content from iTunes is also getting upgraded. That includes any movies you've previously purchased -- so you can redownload the full 1080p version in iTunes or stream the 1080p version on the newest Apple TV.
iTunes 10.6 just got released with support for 1080p HD video on TV and Movies. Accessing previous purchased video content works just the same as it did with music. In iTunes, you click on the "Purchased" link from the home page of the iTunes store, whereas iOS offers a "Purchased" tab within iTunes where you can find content to download. In iTunes, you can set a preference to download in either 720p or 1080p.