Apple recently announced that iTunes in the Cloud was extended to include movies purchased from the iTunes Store. A post on the Tech of the Hub website points out that not only can you re-download or stream iTunes Store purchases, but that some "digital copies" of movies, which you get on some Blu-Ray discs, are also recognized.
So if you have any Blu-Rays with these digital copies, and you haven't bothered to put them in your iTunes library, you might want to do so. If they are matched, you can access them from other devices more easily.
Note: as the blog post points out, not all the movie studios are on board with this yet. So you will certainly find movies that don't match from digital copies.
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http://hints.macworld.com/article.php?story=201203090110435