This KB article:
http://support.apple.com/kb/HT1657?viewlocale=en_US says:
"Can I play my rental on more than one device?
If you download a rented movie on your computer: You can transfer it to a device such as your Apple TV, iPhone, iPad, or iPod if its a standard-definition film (movies in HD can only be watched on your computer, iPad, iPhone 4, iPod touch (4th generation), or Apple TV). Once you move the movie from your computer to a device, the movie will disappear from your computer's iTunes library. You can move the movie between devices as many times as you wish during the rental period, but the movie can only exist on one device at a time."
However, this is from the original AppleTV which had a hard drive. Since the new AppleTV doesn't have a drive, you can't really "transfer" to it, but you can "stream" to it. I would
think you will somehow "move" the movie to the AppleTV, and will actually stream from the internet (the "cloud") instead.