To have a QuickTime movie play on screen during system startup, take an existing QuickTime movie, rename it "Startup Movie" and put it in your System Folder (root level). Now, when you start up the computer the movie plays automatically when the QuickTime extension loads.
The movie plays in a small window in the middle of your monitor screen. No tools are available (such as pause, rewind, sound). The movie window does not go away until the rest of the extensions finish loading. After that, the screen normally clears itself.