I don't know if it's a related problem, but I started getting the same error in a Mac after I switched to Snow Leopard (a few years ago). I haven't played with the latest OS yet (Lion?), so I can say anything about that. When I open the movies create by mne_make_movie with Quicktime I get the black screen Myles mentioned (it has a time component, so I imagine it's a just a video codec issue?), but the videos play fine using VLC.