Praise the Apple gods. If you rent a movie on iTunes in the U.S., you now have 48 hours to start watching and finishing it. And you can watch the movie as much as you want during that 48-hour window. As always, you have 30 days to start watching the movie.
iTunes has previously offered 48-hour rentals elsewhere throughout the world, but the rules were different in the U.S. Up until now, you had to power through the rental within 24 hours of starting it. iTunes movie rentals are available everywhere except China.
I've reached out to Apple to learn more about what prompted the change.