StartUpItems vs. (erm) StartUpItems

Anything in /System is supposed to be the business of Apple and the OS only. Other apps, installers etc. have no business in there. So the /System/Library/StartupItems are part of the default system, or Apple's OS updates, that sort of thing. /Library/StartupItems are for third parties' installed servers, or things like Norton Utils or whatever.

The idea is to provide some organization that's easy for developers to follow, unlike the classic OS's single massive and unorganized System Folder/Extensions. There you had Apples's device drivers, third party device drivers, OS services, and random junk that some developer thought it would be cool to have in his installer, and didn't bother to remove in his uninstaller. So if things start misbehaving after you install something, you don't have to play the old "binary search on the extensions folder" game and risk disabling your video card or something essential like that.