So far, it seems to be that the pre-installation of the Sysprogs certificate is the silver bullet to successfully completing an honest-to-goodness unattended installation. The ONLY problem that I’ve found thus far, is that Windows XP Home does not ship with the certutil command out of the box, and as such, one must install the Microsoft Windows Server 2003 Administration Pack (relevant Microsoft KB article).
The only thing I see as a problem, though, is that the Sysprogs certificate is out of date. Looks like it expired on 24 July?