I had been wondering the same thing. I get iTunes Season passes, but I put the episodes on an external harddrive afterwards. As my music still resides on the computer, but the movies/tv shows reside elsewhere, once I've moved the episodes to the external drive, I need to drag them back in iTunes for them to appear. Lately, I've had the same problem; they'd start playing automatically when I did that, which was annoying the heck out of me.
And I did notice that I had been doing something different when importing. Something that if I didn't do, it worked without autoplaying what I imported.
Tell me, those of you that have this problem, when you import music or videos into iTunes, do you drag and drop on the icon in the dock? If so, that might be the problem. When I dragged it to the dock, it started playing automatically. When I had iTunes open and dragged the songs/videos into the iTunes window itself, it didn't start playing automatically.
I have no idea why it does that, and if it'll work for any of you, but try dragging and dropping directly in iTunes instead of on it's dock icon.
I know I was getting extremely annoyed of having to stop the video every time just to import a few more and having to stop them all. Maybe this will save someone else from being annoyed about it.
what do you try to import that has it autoplay, and how do you import it when it does autoplay? I dragged a few songs on my Windows iTunes, and it didn't autoplay. (though I do have Windows 7 and not XP, if that can make a difference).