From the comcast.com website:
"Basic service subscription is required to receive HDTV and premium HDTV channel requires subscription to underlying premium channel. "
So YES, you can connect your coax (split it) to a QAM capable HD Tuner and get WRCB, WDEF, WTCI, ESPN, and sometimes other stuff comes in. When WTVC and Comcast stop fighting - we'll get that too : )
But, premium channels require their box ($5.00 per mo.) or the PVR ($9.95) and the add on fee for that service (like the HBO package gets you one measly HD channel).
Their HD package gets you access to INHD, INHD2, TNTHD, and a couple of others I can't remember right now...