Creating "Dynamic Masks" in Flash

I was just researching dynamic masks in Flash. Dynamic masks are basically MovieClips that behave as mask and can be controlled with ActionScript. There are some great examples on Sephiroth.it:

Dynamic mask 1
Dynamic mask 2

Basically, these examples make use of the MovieClip.setMask command (I believe Flash 6 may be required – I was reading an older Macromedia article that explains a few pitfalls that can occur when using movie clips in mask layers in Flash 5 or earlier).

Hmmm…OK, one more link. Kirupa.com has a really nice introductory article on scriptable masks.