Over on Daring Fireball, John Gruber has a great writeup on the "fake" startup progress bar in Tiger. Head over there to read all the details about it, but here's the interesting bit. Open a Terminal and type:
/usr/libexec/WaitingForLoginWindow
When you press Return, the "Starting Mac OS X" progress window appears (don't worry, nothing's actually happening). To make it go away, either wait for it to finish, or just type killall WaitingForLoginWindow in the Terminal window. Read John's writeup for more info, including details on how OS X guesstimates how long to make the progress bar last.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050614201205203