Under OS X 10.2.4 (and earlier?), I can bypass the otherwise lovely and glorious sysyem startup chime sound if I first press and hold the F3 key (the "Mute" button -- the one with the speaker icon that has no "sound parentheses" to its right) for a few seconds while I press the power button per usual to boot up my GHz TiBook. Handy when the kids are napping or late at night.
[robg adds: A previous hint explained how that if you mute the sound prior to restarting, it will remain muted at startup ... but this tip is more useful as you don't need to remember to do anything prior to shut down. I tested it on our iBook and it worked as expected; it should probably work on any machine that has a mute key on the keyboard. What's extra nice is that this has no effect on your system sound setting; it will be set at whatever you left it at when you last shut down.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030314015727391