First, Comcast is a big company. It's made up of literally *hundreds* of regional cable operators, most of which used to be owned by someone else. So there's no guarantee your local Comcast provider is using the same equipement or platform as other Comcast markets. But in most areas, it seems Comcast uses Motorola digital cable set-top boxes, with QAM-256 modulation. If you can find a HD tuner card that supports that standard, then you should be able to receive/record the *unencrypted* channels only. Unfortunately, that's probably a small fraction of the total # of digital cable channels on Comcast - probably local HD channels only. The non-local, non-premium HD channels they offer (INHD, ESPN-HD) may or may not be encrypted in your region. The premium HD channels (HBO-HD, SHO-HD, Starz-HD) are almost certainly encrypted.
Supposedly after April 1, 2004, cable companies like Comcast are supposed to comply with the FCC's "OpenCable" mandate, and allow 3rd-party equipment to decode encrypted digital cable signals. I'm not holding my breath, though...
I suggest you check the "Local HDTV Info and Reception" thread on AVS to see if there are threads for your local Comcast provider.