HDTV and In Ground Cable - Does this work?

I currently have a sat dish, non-eliptical, non-HD. I wanted to go HD and was going to purchase a new dish, receiver, etc, etc, etc. I called the local cable company (Comcast/ATT) and was informed their digital cable will pass HD signals, with thier HD box converter. The person explaining this had to ask someone else several times, and I even called a second time, same response and actions from another person. Question is this:

Can one get HD signals from non sat, non antenna signals?

Any insights would be greatly appreciated.
Dan