Wal-mart has deals in place with all 6 major Hollywood studios: Walt Disney, Warner Brothers, Paramount, Sony, 20th Century Fox and Universal.
Download prices will be $12.88 to $19.88 on the day of the DVD release. You can choose between three resolution formats. PC Format: 640 X 480 maximum resolution, 30 fps, 1,500-1,700 kbps average bit rate, Portable Format: X 240 maximum resolution, 30 fps, 500-810 kbps average bit rate, PC & Portable: a combination of the above that requires 30% more storage.
The portable video format is not compatible with the Apple iPod video.
Because pricing is pretty close to disc pricing it is not really interesting. I like Amazon's unbox focus on short-term rental like downloads for a couple bucks better. If I go buy a full movie I want the disc.
More details in this NY Times article and on the Wal-mart Movie Download site, which is up and running in beta.