Replacing the ol' green backdrop with a background. How?

I've seen a lot of documentations in editing videos, where they shoot the video with a green backdrop, and replace the green backdrop with a video/background during editing.

Does anyone have an idea on how to do that, specifically on a Mac? What application should you use, and what you should have? Thanks in advance.

It can be done in Final Cut Express under effects keying.

Any good online tutorials on it?

Try fce tutorials.

Motion does it (or is it shake?). Check either one of them, i'm pretty sure I saw a video of exactly that.

Oh, Final Cut Express. Does Apple have a trial for that? I wouldn't want to waste my $300 on something I haven't tried yet.

if you have iMovie HD6, I believe there are plugins that allow you do color keying (or chromakey, as it is referred to often). Check www.geethree.com for info.