My MacBook Pro was playing up; maybe an external disk got frisky, maybe some projector somewhere, I don't know. It's working again after four (4!) visits to the Apple Store. First up: Bondi was totally useless. They reinstalled Snow Leopard and sent me on my way. The CBD store decided the DVD drive was busted, but when it came back it was sicker than before, with scanlines on the LCD and a complete refusal to start. They took a week to replace the guts and it has been OK since. All of this was free under AppleCare; that has about six months to run. I'm not very impressed with the working knowledge of the geniuses. I did enjoy being without it for a week though.
For future reference: my retail copy of Snow Leopard is too stale to boot this machine. The grey disks that came with the machine crashed when restoring the Time Machine backup (!). Fortunately an up-to-date Snow Leopard got things back.