I think I have seen what you are talking about, but I don't know of anyway to eliminate it.
I use a Sony TRV-310 (Digital 8), and I have a flashing black line or border at the top of my frame that I see from within iMovie and when I export as QuickTime movies but not when I export back to the camera and view on my tv.
I have noticed that most of the iMovies posted on the Apple site, in the iMovie theatre and Apple Masters page, have a black border on the left edge.