The machine did, indeed, fail to power up. I checked all of the connections, which appeared to be solid, before deciding a peek inside would be a good idea.
I've seen photos, and been near plastic-shielded machines with the metal side off, but this was my first time actually inside a G5. It's nice and roomy, and very nicely laid out. It was fairly quickly clear that the NVIDIA graphics card wasn't seated properly, so I pushed it back into place. Must've been jostled loose in transit. Hit the power button again, and the machine came up happily. Tom got through the initial setup, paired his keyboard and mouse to the Mac's Bluetooth feature, and let me get on my way.
Oh... and the 30" Cinema Display is even more impressive on a desk than it was surrounded by a crowd of people pawing over it on the day it was announced!