As seen at the Windows Vista Team blog:
We heard you loud and clear when it came to discussing our plans to include a default start-up sound that plays as you boot-up Windows Vista: you said you wanted to have the opportunity to mute it if and when you felt it necessary. Your collective voice truly made a difference in this matter, as we’ve incorporated a way for users to turn off the start-up sound when it’s more appropriate to boot silently — be it when you’re mobile, at home or in the office.
Thank-you!
Of course, if you’re going for true coolness, it would be nice to have the ability to automagically set sound scheme, volume levels, etc. based on where you appear to be…but that’s something that can presumably be done with an appropriately-written script.