Well, all my PAID, bought on iTunes movie poster art is now missing as well in iTunes 11.1.2. My largeish personal movie collection, wherein I hunted down the poster art and manually added it... all still work. Further, I note, in moving one of my processed films between Macs, the cover art is embedded in the H.264 movie file itself. It cannot go away. As a test of this, I transferred one of my files to a PC notebook of a friend, and even iTunes on the PC sees the cover art fine.
This tells us, code wise, movies from the iTunes Store do not have embedded, rather linked poster art assets, and thus, the link within iTunes between movie and poster art has been severed in this release. This is why an incremental update to iTunes can magically restore all the missing cover art.
A note to Apple Programmers Imagine were I Steve Jobs telling you this. (Oh, I am THAT guy who caused a rewrite of the Apple Store. Remember THAT?) Here is the permanent fix to cover art issues: In a hidden, locked (after edit) Unix file stored safely somewhere, you just have iTunes use an OUTBOARD FILE which simply contains the link data for the art. THEN, when ANY new version of iTunes comes along, it just reads that little file, and NO USER is ever left without art. The file is opened and modified as needed, and just locked up after each change.
Further, the poster art is not searchable by name on your computer if you wanted to do a manual add to permanently fix the problem.
From experience, I can tell you manually adding movie poster art, which iTunes then embeds, is painfully slow even on a fast machine. That is because a huge file must be rewritten. This is why Apple may have reticence - keeping user experience positive.
Further, Apple may be using a direct screen dump technology for the poster art, which puts a graphic onscreen without using further memory. Large collections could tank a lesser machine if this was not used.
Well, at least my movies look good in Apple TV.