If you have a high-resolution system, you can definitely hear a difference in brand new, unbroken-in cables and cables that have been properly broken in. I don't presume to be able to explain the physics of exactly what's going on here, and it may vary considerably from cable to cable, given the great diversity of designs, but anyone who has had much experience in this realm can confirm this; it's definitely not audiophile neurosis or hocus-pocus.
Within the past year I had the experience of ordering a complete new set of Purist Musaeus cables (two sets of interconnects and one set of speaker cables). When I initially hooked them up, I was somewhat disappointed. The manufacturer stated that the cables required a minimum of 100 hours of burn-in. Well, I might once have been skeptical of such a statement, but I can confirm from personal experience that after 100 hours, this set of cables sounded quite audibly better--smoother, better balanced, with better resolution of detail--than it did when brand new. Later I ordered an additional pair of new Purist Audio Musaeus interconnects, and the same experience was repeated: they sounded quite audibly better when they had completed break-in. This experience has been repeated by so many audiophiles that it is commonplace and well accepted by those who aren't so blinded by theory or what some textbook told them that they won't believe what their ears tell them.