There was a domain register I believe it was networksolutions that was offering 100 year domain registrations but they don't actually register it for 100 years. They would only register the domain for 10 years at a time and would renew every 10 years after that for 100 years.
That's assuming that the domain registrar would still be in business 100 years. At the moment you can only register and renew domain names for 10 years.