
They explain about compatibility among Apple's movie editors:
They don't say what happens if you open an iMovie project in Final Cut Pro, but I imagine it's the same as opening it in Final Cut Express.[Final Cut] Express will open iMovie 3 projects, but in doing so, you'll lose transitions, titles, sound effects, and custom audio levels. A Final Cut Pro user can open Final Cut Express projects and have full Final Cut Pro capabilities; however, you cannot open a Final Cut Pro project in Final Cut Express.