The United Offer was on a card mailed to me. It says you do not have to register and "triple miles will be credited automatically to your Mileage Plus account". The only condition on the card is that "Customers must provide Mileage Plus number at the time of reservation and/or rental to earn bonus miles". However it does say "This offer is non-transferable". I wonder if that means it is only good to people who receive the flyer. I will try to find out
Paul