As I understand it, YouTube converts whatever type of movie you give it into Flash (.flv) format. So iMovie would be a great program to make a short film like you see on YouTube! If you really need it in Flash format, you can convert it with ffmpegX (which isn't all that great of a program, but gets the job done) or if you aren't worried about quality, upload it to YouTube (thereby letting them convert it) and then download it.
When you visit Youtube their are short flash adverts on the page at the top and on the side of the page. Many web pages do this. Something cheaper that cs4.