Well, but just to elaborate, for those not familiar with it...
I can give an example from Vegas 7. Say I want to have video filling in the letters in some CG text overlaid over my regular video in a project.
I tell it to make the CG text a mask, and make the CG text mask a parent to the background that I want filling in the mask. Then that child is only tied to the parent layer, so that the alpha channel from the parent layer can still be overlaid on your main video.
Sounds complicated, but it's a quite powerful feature. Ah, well, yeah, we need the ability to set something as a mask, too.
Looks like we need a SpeedEDIT feature requests forum.