Apparently Microsoft doesn't think you might want to customize your startup sound
From the looks of things, Microsoft seems to be so smitten by their marketing that they believe that you'll always want to hear the default Vista startup sound, no matter what.
Here's a portion of a quote (from the Scobleizer link) from Steve Ball, the program manager for audio video experience (oops, I mean excellence) team at Microsoft,
"The bottom line is that the rumors, stories, speculation about the new Windows Vista sound are true," Ball said, "but with a number of extreme qualifications."
The current plan, he tells me, is that there will be a pre-wired sound that plays when the system is ready for you to logon. This is the plan of record for quite a few months.
You can do other things with your attention and your eyes during cold boot without feeling like you have to watch and wait.
This will be a non-customizeable sound, and that’s been part of the plan for Windows Vista for many months, he said".
The overwhelming number of user comments on the scobleizer page are clearly in opposition to this idea. And it seems to me that most of those commenters aren't as attached to the idea of customization as the visitors to Wincustomize are.
Even if I didn't want to customize my PC, I would be unalterably opposed to this kind of thing. Being someone who enjoys customization, I see this as a much worse proposition.
Steve Ball mentions that they are 'listening' to these kind of comments, but the statements that he makes (including a couple in the comments section) indicates that Microsoft is dead set on providing this 'functionality'.
If I get Vista at some point, and it has this issue, I will look for a hack to disable it, as much as I hate the idea of those kind of hacks.