Apparently in the new version of Vista, the startup sound may be hardcoded into the system. I think this is a very interesting thing, mainly because there is a battle between customization and keeping a standard face. The reason for having a boot sound has multiple uses, the blogentry talks about some of them, but I don't agree with them. In a corporate environment, when fifty people all turn on their machines around the same time, hearing that boot sound doesn't really tell you that your computer is ready. It tells you that someone's machine is ready.
Also, I like customizing my machine. I use thunder as my bootup sound, just as I use a cat's meow in my IM client to tell me when someone says my name. It is a sound I picked and it is a sound I can listen for because it is the sound for me.
I don't like the Mac bootup sound for the same reason. Yeah, it is a branding aspect, but I know what operating system I'm using. I'm usually very sure of what operating system and computer I'm using, I don't need to be constantly reminded of that fact.