I just bought a used iMac and the guy already has registered with apple. How do I re-registered it with my information so that apple has the right owner on file??
I'm not quite sure why people even bother. The warranty is attached to the serial number, not an individual. Upon your first support incident, the AppleCare Rep. will add the computer to your support profile.
The only time a "change of ownership" is even necessary is if the unit has an AppleCare Protection Plan warranty. And even thenAppleCare can transfer ownership over the phone despite what the AppleCare agreement says.
AppleCare agreement states the original owner must send a letter to AppleCare requesting transfer of ownership, etc.
If the previous owner reinstalled the OS it will take you through the registration process at startup. That's what happened with the Powerbook I bought last year.