Looking at Greenscreen Software for Macs
My daughter is a huge fan of movie-making and her next foray is into the realm of green screen (or blue screen) movies. She's talking about getting a loft bed to make room in her bedroom to put up the screen. That's a clear vision, I have to say.
So I did a little digging and I have to say that there seems to be a lot of plugins for the older versions of iMovie - you take two clips, one that's the background, the other is the screened clip, and then layer the second on the first. It's a nice way of doing things, but as I've also found out, Final Cut Express ($199) does all this and a whole lot more. So that may explain why I haven't seen any plugins that have kept up with the latest iMovie.
Of these, she seems to be most interested in eZedia's eZeScreen plug-in for iMovie 6 (or later). It's a Universal Binary, and says it'll work with anything after iMovie 6, so we'll have to see what happens when we get the demo and try it out.
I've also seen Norrkross Movie, which is sort of an iMovie replacement, and it's not bad looking for $59. It's got all the things she's looking for, and is probably the place to start as it's a nice entry point for what she's doing. If she keeps up with this, Final Cut is a reasonable alternative, but it's after she's kept with it.