CYaBro: Forget the 75% who will be able to get FreeviewHD.
What about the rest of us who live outside the "nine main centres"?
When will we get free-to-air HD TV?
Their is planned expansion from 75% - 87% in the future. This is pretty close to the estimated ~90% of the population who have access to the existing analogue broadcasts.
If Freeview do eventually deploy DVB-S2 via satellite they could broadcast in HD but there are no definate plans to do this so the simple answer to your question maybe never.