The guy behind the desk repeated my diagnostic steps, and went on to remove the video card and the RAM. His best guess was that the motherboard's dead.
"Tycho" from Penny Arcade was talking about how pleased he is to be able to run his choice (or all three) of the popular OSes on a single box, and I entirely agree. Having options is a good thing; it keeps the vendors honest - nobody likes a monopoly (nobody likes somebody else's monopoly, I mean). So, kudos to Apple for having a laptop that doesn't chain you to a single OS vendor. If only you could go back in time and have a tower workstation where there are a variety of motherboard manufacturers, providing market pressure and lowering the price.
Donations to the "Dave needs a heavy piece of machinery for his iTunes fix" fund can be made at the door. They're tax-deductable if your accoutant's not ethical.