I do Vjing (semi-pro) and many of my clips have letterbox bars in a 4:3 box. I recently needed an easy way to remove them and just have the image. After a bit of trial and error with masks, I worked it out ... and of course, then I found this write-up which explains it all. From the author's comment to his original post:

What you want to produce here is a black box with a white border, as seen in the bottom left hand corner of Fig. 3. It will look a bit like a film slide. (You can also make a white box with a black border and then hit the Invert button in QuickTime Player, but I digress.)

When you insert this black-on-white graphic into QuickTime Player as a mask, QTP will hide the parts of the image that lie under the white areas.

This has proved to be very useful indeed. It's a great way to easily crop out unwanted portions of a movie.