I’ve been having the same issue. The image looks great in FCP and in Quicktime, but it looks dark and contrasty when its played on a Flash player like Mpeg Streamclip or Vimeo.
I don’t know why the players are so different, but it would be nice if I could calibrate my viewer in FCP for Flash, since Vimeo is my video’s final destination. Let me know if you’ve had any luck with this. 🙂