This has been the case with the iPad and the Apple TV. As the iTunes Store terms and conditions points out, movies rented "using the Apple TV or iPad may not be moved." So this isn't unprecedented, and it is mentioned on page 99 of Apple's iPhone User Guide for iOS 4 Software (PDF link): "On iPhone 3G and iPhone 3GS, you can transfer rented movies between iPhone and your computer. On iPhone 4, you can transfer rented movies between iPhone and your computer only if they were rented in iTunes on your computer. Movies rented on iPhone 4 cannot be transferred to a computer."
This must be due to a resolution issue, iLounge points out, as the iPhone 4's retina display at 960 x 640 falls short of full 720p, and barely beyond standard resolution of 854 x 480.