Pretty good question, but alas the digital world has to bear under a huge set of strange rules and protocols.
First of all most digital output is set up in such a way that you cannot actually record it instantly. A huge exception to this is unencrypted FireWire digital media, but you cannot "just" record HDMI for instance.
The actual TV signal is a huge bandwith set with encrypted data. On the cable (or dish) all signals are coming instantaneously. You need a decrypter/decoder to take a little of that bandwith and reform it into a single digital signal. (This is what that stupid settop box does. If you switch channels, it takes another piece of bandwith).
So, after that source of all channels has become one source, you could actually record it digitally. Now to find a HDTV digital dvd recorder that can record straight from the HDMI cable...
And recording digital still means you have to choose a format and a drm strategy. (Seen from the company's point of view).