Well you can make an HDTV PVR for a little over $1000. There are two cards I know of. The HiPix and the AccessTV.
These however only record an ATSC bitstream off a tuner. That is good for off air and for those lucky enough to have an Echostar 5000 receiver with HDTV modulator.
Recording analog HDTV on a HOME computer is out of the question at this time. Remember that analog HDTV is decompressed. That equates to a data rate of 160MB (megabytes) per second. There is a product manufactured by Aja Video that will place HDTV on a PCI buss but that doesn't mean the computer can handle it in real time. The card is mainly intended for still capture and display. It also uses a digital interface so an analog convertor would also be needed, you are at $12,000 already.
Compression? Yes, that's the answer but at this time an ATSC type MPEG2 compression unit is a six figure device. Will home computer ever be able to capture real time uncompressed HDTV and store it? Of course. How long will it be before 10ghz processors and 10terabyte hard disks selling for less than $100.