Eric,
While I think all of your systems are worthy blueprints for me to build to sell to home office people as a side business, none of them are acceptable in any way whatsoever for corporate use. I use a homebuilt PC at home, I'm building a dream Athlon system for a friend, but I neither use homebuilts at work nor would I ever recommend them.
Corporate PCs require professional vendors with return policies, long term warranties, 24/7 phone based tech support, overnight shipping of replacement parts and systems, and a trusted reputation. While management is usually held to blame for this, that is only a partial explanation. I don't know where other companies find personnel, but I have never found a help desk employee that I would trust to build a PC from scratch or maintain one based purely on a list of components. In fact, in terms of building a PC from scratch, I know many upper level IT professionals who have never touched a jumper or dip switch.
The economics of homebuilt systems in your articles also don't take into account the productivity costs of ordering parts, maintaining a stock of parts, and physically building the PCs. Nor does it take into account the cost of an ergonomically correct workshop in which to do so. I'm comfortable building a PC on my living room floor, but that's simply not appropriate in a work environment.
In my opinion this relates directly to the question of AMD chips. I have never purchased anything other than an AMD chip for a home system (and given the recent defeat of the P4 at the hands of a lowly K6-3+ in a Pi calculation benchmark, it doesn't seem likely that I will do so any time in the near future). However, I have never purchased an AMD based PC for business use because no major vendor has offered mainstream business PCs with anything but Intel processors.