Hey guys. I've been watching this thread. Some good points.

Availor, a 20 frame Intro say set at the default 12 fps will complete in less than 2 seconds. I don't how large your final movie will be in megabytes, you mention 1 mb, but 2 seconds isn't long enough to download that. You'll probably want a preloader, especially if someone is on a dial-up connection.

On another note, as something to consider, using MC's as Buttons. I've done this several times and it's nice because I can have more states than a button's three.