These are currently available from HP for as little as USD $429.99 (bare bones), including a rebate. There's actually a choice of 4 different AMD 35W CPUs, plus memory upgrades to 2G, among other upgrades, all at extra cost.
HP s7600e Series
The mini-ITX MB form factor is identical to the VIA EPIA series and should fit in any mini-ITX case. Even though the boards may vary slightly in overall size, there's an industry convension that defines hole placement for all MB forms.
It's a pity that you can't just buy the MB with CPU only, but that could change. For example, early shipments of Windows XP Media Edition were also limited to HP products, but after a year or so the OS was released to the general public.
For comparison, the latest VIA EPIA MB with the latest C7 CPU retails for about USD $230. For another $200, HP is giving you a case, RAM, optical drive, etc., along with a much more powerful AMD CPU. If you can recycle the extra parts or dump them on eBay for, say, $130, you've got a much more potent combo for $300. Considering that the Albatron 754 mITX
MB sells for $300 without a CPU, it's not such a bad deal. In fact, if you wait a few months, you'll start seeing preowned entry level HP units on eBay at a substantial discount. On the other hand, you could also keep the case and periferals and replace the PSU with a picoPSU and outboard transformer. It will add to the cost, but if you can live with one expansion slot, it would still be a very powerful, cool, quiet, and energy efficient rig for a reasonable price.
Interestingly, there's also a Celeron M version of this series called the s7600y:
HP s7600y Series
Disclaimer: I have no affliation with HP, AMD, Intel, etc.