From this thread (Google will require you to login):

The G1 is aimed at end users, not system developers. For user security reasons the G1 will only accept properly signed system images. I'm not sure, in this case, who 'owns' the key, whether it is the carrier or the manufacturer, but one or both of them handle insuring system images are signed.

Cheers, Justin Android Team @ Google

Surprising? Not! Yet another chimera Google wanted people to believe in. And, people, don't point to the OpenMockup. The damn thing does not even do quad-band GSM, let alone 3G, non-withstanding the "port" is not done yet.

(Thanks to mako for the hint)