Indeed, all US HD services are delivered via premium satellite or cable services such as Dish network, DirecTV, Comcast etc.
They are all pay subscription models. The US has a lot of HD pay for view sports aswell.
Incidentally, George Bush signed the Digital switch off Mandate yesterday, 2009 is the analog switch off deadline. Theres not really any digital terrestrial service in the US (some pilot schemes), so I expect that airspace is going to be a great opportunity for the TV providers to rollout digital terrestrial HD.
This bodes well for the UK getting HD freeview at some point in the future.