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That’s never been option. If you purchase your rental on the Apple TV, you can’t transfer it back to iTunes. If you purchase your rental on iTunes, you can transfer it to the Apple TV, iPod and iPhone (one device at a time).

It’s because the rentals purchased through the Apple TV are of a higher quality and bit rate (Apple touts them as near DVD quality) than the rentals purchased through iTunes (which are encoded for the iPhone specifically).
 
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