This is easy to do with a Mac. You do need additional equipment, as well as the Macintosh programs iMovie and iDVD. The additional equipment is an analog-to-digital converter, such as the ADVC boxes sold by Canopus. As mentioned in the other posts, most PVRs have analog video and audio outputs. Both my Tivo and Comcast HDTV DVR (Motorola 6412) have such outputs. Connect the analog outputs (using either composite or s-video) to the AD converter, and connect the converter to the Mac with a Firewire cable. The Mac then thinks the AD converter is a digital camcorder, and the digitized video can be imported into iMovie, edited (remove commercials, etc.), and burned to DVD using iDVD. Note that the video uses lots of hard disc space--about 12 GB for an hour of video.
There is a way to record HDTV directly to the Mac using the Firewire port on the Comcast DVR, but it's not too practical. Not only does it use gobs of hard disc space, but it's not easy to record to DVD at this time.