>Anyone know this? Every MSDN article seems to assume you already know this, but no one here does. Is it from the boot.ini after the backslash on the "default" line, maybe? That would seem to make sense...

It's the last element (after backslash) in the ARC path in the '[operating systems]' section. "Default=" points to the ARC path of the default OS. See BOOT.INI and ARC Path Naming Conventions and Usage mskb#Q102873 and http://multiboot.solaris-x86.org/iii/2.html.