Unlike digital cable, broadcast digital TV (ATSC) is not encrypted. The raw transport stream is easily saved to disk, recovered, and transcoded.
There isn't a good way to work "natively" with digital cable. The least unattractive option is to use a set-top box for decryption and feed the video stream to a capture card. At this point, though, the only capture cards you can buy are standard definition. You'll also need to use an IR blaster to send channel changing commands to the set-top box.
Let's hope that the broadcast flag's chilling effects won't stop high-definition capture cards or a CableCard capture cards!