I didn't realise this would be so exciting, because it has been on the radar for quite a while now. It is only in trial now and volunentary and goes live on January 12th for NON US CITIZENS who formally had to fill out the green I-94 form before entering the US.
Similiar to the Australia, which has an online only, but in a lot of cases the travel agent fills in the information, though you can do it yourself. Third parties are allowed to fill in the information, and most people expect the service from the travel agent and as people are finding out, the airlines are notifying people if they booked directly with the airline.
Just to point out, the only way to get a UK Visa from the US is by applying online...