LinuxMCE is using Amazon to lookup the movie information, downloaded cover art, and all that good stuff. MythVideo uses imdb.com for this.
Looking at a search for "Matrix Revolutions" on both as an example:
Many returns, with no real 'good' selection, about the best cover art is a PAL version, but lacks a good description of the movie:http://www.amazon.com/s/ref=nb_ss_d/103-6001351-0207017?url=search-alias%3Ddvd&field-keywords=the+matrix+revelutions&Go.x=0&Go.y=0&Go=Go
and one return on imdb.com has the perfect cover art and a very good description.http://www.imdb.com/title/tt0242653/
MythVideo also allowed one to directly enter the ID (0242653) to pull down the information. The main difference here is amazon.com is trying to sell a DVD, or a blue-ray, or an hd-dvd, or a VHS, etc.. Each one has it's own entry, for a single movie, all of them being slightly different, but never having a good outline of the movie. imbd.com doesn't care how many times it was released, it's all under one single entry.