Hidden HD Channels?

I have comcast basic cable (and by basic, i mean i have discovery channel, qvc, etc. but not disney channel, mtv, comedy central, etc.) and today got an hdtv. I was expecting to have to change the plan and add the HDTV package onto it, and get an HD receiver. But when i set it up and went through the channels, I already had HD channels for ABC, NBC, CBS, the CW, FOX, PBS, and a few others. Is this a mistake, or is were the channels always broadcast through, and i just didn't have the right television to receive it?