How important are speaker cables to the sound emanating from the speakers?

rzado may be right, but the first specialty high end cable I remembering hearing of (via Gordon Holt at Stereophile) was the Fulton Brown, in '75 or so. I may be mistaken about the exact timing, but I seem to recall reading about the problems with the Polk cable (it's extremely high capacitance caused many amplifiers to become unstable and oscillate) after the Fulton was introduced.

Roger Sanders makes a speaker cable optimized specifically for Electrostatics. He explains the difference between the electrical characteristics they present to a power amp versus that presented by non-ESLs (the voltage versus current paradigm), and the related characteristics the cables should have to optimize the amp/speaker interface.