Those don't look like G5's or even G4's to me. Those are probably POWER6 based which means they'll never be in a desktop machine.
According to a reading of the chip's
specs, plus a reading on the
Power ISA, the chip is a 'system on a chip' (CPU core, plus what we normally call the "northbridge", consisting of memory controller and I/O controller,) that is fully compliant with Power ISA v2.04; which is the successor to Power ISA 2.03, which the PowerPC 970 (aka "G5") is compliant with. In addition, Power ISA 2.03 is a derivative of the PowerPC ISA 2.02. Essentially, "PowerPC" was rolled back into the greater "Power Architecture", with the G5 being the first mainstream "PowerPC" processor to be released under the new definition.
(Chip specs link goes to a Google cache since PA Semi's website is down at the moment.)
As for never seeing light in a desktop, that is almost assuredly because nobody makes PPC desktops any more. These are for either HPC clusters (like the linked 9-CPU box,) or for embedded uses. Plus, at 13W, this thing could very easily power a "PowerBook Air G5" that has better battery life, and better performance, than the existing MacBook Air.