followup
I should add that I found an easier method for special cases (assuming only QT Pro, dedicated tools like previous post are probably more streamlined still).
During Export, if you use QuickTime Movie for the format, and click options, then click "Size", choose the target dimension, and then check "Preserve Aspect Ratio", and then "Crop" from the drop down, it will scale the movie to fill the image and then crop off what doesn't fit. (So for instance, choosing 640x480 on a 1280x720 image means it scales down to 480 high and then chops off the pillars to "pan-scan" HD 720P down to 4:3 SD ).
FYI, there's a similar size/aspect/crop sequence available if exporting to "raw" MPEG-4 instead of QuickTime Movie.
Of course, this only selects the middle of the image, the masking method lets you grab something off-center.