Apparently iTunes 10 thinks all mp4 files are music videos or something, and so pops up a separate window, containing just a still image with the artwork. Or worse, it happens randomly (I often play in random order). Or worse, it switches to full screen (I have "Play music videos in full screen"). Solution: Setting "Play ...in iTunes windows" or "...in artwork window". Forget about fullscreen mode. iTunes 9 just played along nicely in the background without annoying anyone.