Get a better idea of what you can do with iMovie 09, such as chroma key imovie, chroma key video backgroundsand chroma key images. I have done some green screen work with iMovie and I can tell you it is really easy to do. Much easier to do in iMovie than it is in FinalCut Express for example.
I did have a bit of a thing against iMovie because iMovie 08 was so crap, but over the last couple of days I have been looking at some screencasts showing some of the features added to the latest version. The precision editor really makes a big difference and makes iMovie useable without the traditional time line.
Looks really easy to move a clip left and right in relation to the next clip and you can have the transition just where you want it. You can use the precision editor with the transition too so that you can alter the length of transition if you want to.
I have even seen a screencast that shows it is possible to do green screen and picture in picture in iMovie.