how do i hide sensitive information from quicktime screen recordings?

Plan A

1. Screen Record all you can without the screens in question
2. Take screenshots (stills) of the remaining
3. Edit the images in your favorite Image Editor that allows for eraser or filled rectangles
4. screen record any mouse/cursor action over the stills
5. Splice 'em back together when you make your final movie in iMovie, FinalCutPro or whatever Video Editor you will use

Plan B

http://www.adobe.com/products/director.html <[ formerly Macromedia Director - my stock and trade ]

VERY robust for 2-D animation - which is all you are seeing when you interact with a device's display anyway. Many a tutorial authored in this App - as well as many a robust custom app as well - Director.App GUI & functionality was made WITH Director itself and plays in the "Engine" (Player can be freely distributed)

ÇÇÇ

QuickTime Player 7 Pro could be used to position a black rectangle and scale it to cover the area.

QuickTime Player X doesn't have this option.