Every time you boot up your Mac you hear the familiar boot up sound. But it does get annoying after a while. Disabling it is quite easy, all you have to do is mute your volume. But you have to remeber to do this every time before you shutdown. This is a pain. There are methods in which you can get around this. One uses a preference pane. The other involves creating files that run on boot up and shutdown.
The best and most easiest method is to use a preference pane. If you head over to StartupSound.prefPane, you can download the preference pane. Once as you have installed it thorugh the installer. You can modify the sound value for the start up sound. To disable you just press mute. When you boot up again the sound will be gone.
The other method, which involves using a shutdown and boot up script is located at Mac OS X Hints, the post is here. By adding a file it achieves the same method. The only problem being is that you have to have the knowledge of creating files within Terminal. I prefer the Preference Pane method as it is a lot easier.