The signal with digital is bi state, however the timing of the signal is not. This is why with digital audio, cables do make a difference. Jitter can affect the quality of the sound and jitter will be affected by the cable signature amongst other factors. Internal reflections and interfaces can also introduce jitter, especially in optical cables.
However, whether digital video is affected by timing is something that I have not found out. It it is a digital signal for amplitude but, like digital audio, if it is analogue in the time domain, then the cable will have an affect on PQ. However, if timing, within a normal window does not affect the picture, ie as long as all the information for a frame is received within a certain time frame, PQ will be unaffected by jitter in the signal.
There are reports of cable quality affecting PQ, even from reputal sources, but so far, this has been only for longer lengths. This is also backed up by manufacturers who will only certify their cables at certain resolutions for below certain lengths.
For me, the jury is out and I will be sticking with my generic HDMI->DVI-D cable. But I will be borrowowing a more expensive cable in due course and make my own decision. Just like I did with digital audio where I ended up auditioning a selection and buying the ones that produced the sound that I liked best.
-Ian