Wow, ouch. I just recovered from what the nice people at Apple called, clearly impressed, a "very rare" type of complete system software failure. Neat! I got to experience the joy of a total system reinstall. On a Mac! I didn't lose anything, thank goodness, but it more-or-less ruined my day yesterday. How much fun is it to totally reload your entire iTunes library? Plenty!
A lesson learned: when you replace your iMac, keep the old one around, sulking in a corner until it can be of use. I was able to Firewire them together and retrieve everything accumulated over the past three months before doing the reinstall.
Next purchase: Time Capsule.* You bastards.
*Actually, probably not. But I probably will get a new backup drive and use Time Machine.