With no fanfare during the recent iPad / Apple TV event, Apple has quietly rolled out the availability of Digital Copy for the Apple TV via iCloud.
The capability is similar to that of UltraViolet, the digital copy mechanism that accompanies the purchase of most Blu-ray DVDs. As with UV, you use the Digital Copy unlock code to stream an HD copy to your updated (you did update your Apple TV, didn't you?) Apple TV and authorize a downloadable file.
Tech of the Hub notes that they were able to watch a previously activated Digital Copy of Star Trek in 720p, with the assumption that the copy would appear in 1080p on the new Apple TV. The movie was also available on an iPhone running iOS 5.1. Other movies don't appear to be available in HD; the site mentions that Kick-Ass was just a high-quality standard definition digital copy.
Not all movies will work in this way. There are licensing issues with both Fox and Universal content right now, so you won't be able to get your Digital Copy for movies from those studios. Apple has a page that describes the process of adding your Digital Copy to iTunes for those who want to try it with an updated Apple TV.