I have no idea what's going on, elsewhere, but my cable company (Comcast) does not charge extra for HD, per se.
First of all, though, remember that there may never be a time when all channels will be HD. I think some channels will remain in standard definition -- although all channels will be digital.
Originally, I had basic service, only, which meant the analog channels, and no set top box. After purchasing an HD set, I decided that I'd like to get the local broadcast channels in HD, so Comcast gave me a box which gave me just that -- the same analog channels I'd been getting, plus the local HD broadcasts (as well as the PBS standard def sub-channels). Since they had to give me a digital box to do this, they charged me an additional $5 a month for the box. I suppose we could get picky, and say they're charging for the HD channels, but, there really is no way they could deliver those digital HD channels to me without a box, since ATSC is the over-the-air digital format, and cable uses QAM. Therefore, I really do consider the $5 to be the rental cost of the set top box.
Later I decided to add the basic digital package which includes about a dozen, or so, HD channels, such as Discovery, Food, ESPN, TLC, etc., etc. I pay the same thing for that package that someone who has only a standard def TV pays, but I get the HD channels, as well. (Actually, those with older sets get those channels, but they're downconverted to standard def.)
So, ARE they charging extra for HD?