My point is that it's an unnecessary word. A word already exists to describe that concept - "disembark," as Macleod says. I'm all for new words to describe new concepts. Of course Chaucer didn't have a word for "internet," because he didn't need one. Likewise there's no need to create a new word to describe "disembark" when there's already a perfectly good word that serves the purpose. After seven years of living in the States, I have to say my impression wasn't that it was a dialectal difference, but rather ignorance of the fact that the word "disembark" exists at all.
Well, you say that, but "embark" comes from Middle French "Embarquer", with Em- being from "En" meaning in, and "Barque" meaning ship. So you're actually saying you're boarding a ship, not an aircraft, when you embark. (Barque came from the Latin Barca, meaning ship.)
Thus, dis-em-bark would mean to get off a ship. I can certainly see the point in having a specific alternative for aircraft.
(Of course, it's also true that you "embark" on a journey - this is a broadening of the original meaning, and lends support to the thought that we don't need any such alternative, but some people just prefer to be specific about their mode of transport.)