For launch day, they offer about 1,300 movies and shows, with shows going for $1.99/episode and movies going for either "rental" prices of around $3.99 or "buy" prices at $7.97 and above (I see a recent film at $14.99).
They have a FAQ and it mentions that stuff you "buy" is always stored at Amazon and you can re-download whenever you want, but I don't see a definition of how long a "rental" window lasts (I see it is only 24 hours on the purchase pages). Additionally, there's a really bizarre "Pay-TV Blackout Window" where stuff you bought and viewed before can't be re-downloaded for a short time when it is playing on Pay Per View. That one is really weird, considering no sane person would re-buy the same movie they already purchased at Amazon months before.
I bought an episode of Studio 60 and the Borat movie to test it out. In about an hour, I should have both on my Now Playing list, and I'll check them out today and make a follow-up post about the video quality and experience. This looks pretty cool so far and I love that new movies on the day they are released to DVD will be available via Unbox. No more trips to the video store! (thanks Megazone).