Apple has tightened its iTunes rental T&Cs, removing a loop-hole that allowed TV shows to be transferred between the iPad and other iOS devices. The new agreement, paidContent spotted, now refers to generic "content" rather than differentiating between movies and TV shows, and basically means that anything rented from your iPhone, iPod touch, Apple TV or iPad is now only available on that device.
The previous T&Cs read "Movies or TV shows rented using your Apple TV, iPhone 4, or iPod touch (4th generation) may not be moved. Movies rented using your iPad may not be moved," differentiating between the iPad and the other iOS devices. That has now been updated to "Content rented using your Apple TV, iPad, iPhone 4, or iPod touch (4th generation) may not be moved."
If, however, you purchase a rental on your computer, you can then transfer it to an iOS device for portable viewing. Attempts to go in the other direction will see the content wiped after a restore. There's also speculation that the change in terminology is to encompass games and other titles, as the App Store and iTunes extend their scope.