From the
FAQ:
Can I play my rental on more than one device?
If you download a rented movie on your computer, you can transfer it to your Apple TV, iPod, or iPhone. 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 be viewed on one device at a time. If you rent the movie on your Apple TV, it is not transferable to any other device and you must watch it on your Apple TV.