I'm not sure what's behind it, but minutes after I received a free email coupon from the iTunes store to rent a free movie from iTunes, I started hearing from others who have received the same offer. My email has the subject heading of "Your Free Movie Rental On iTunes." It came to the email account I use for iTunes purchases, and it appears to be good for any iTunes movie rental, whether HD or regular video.
The way iTunes movie rentals work, you pay for the movie (or use a coupon) and select it from around 8,500 possible rental downloads. You have thirty days to start watching the movie, and once you start watching, you have 24 hours to finish watching it before the file is automatically deleted from your drive.
It's not a bad way for Apple to promote its movie rental service, especially given the popularity of NetFlix and the new Hulu Plus service, both of which are iPad friendly.
This will be the first movie I've rented from iTunes; I'll let you know how it works.