I'm not a cable skeptic, but I trend, with Doug and Al, to being a break in skeptic (with the already noted exception of transducers).
The reason is that the experience of components improving with time is predicted by several well known psychological effects, which collectively suggest that much of the "break in effect" is mostly what people here have been calling "psychological accomodation."
1. The "mere exposure effect": people tend to prefer familiar stimuli. Thus, the more you hang around your component, the more you can, all else equal, be expected to like it.
2. The "mere ownership effect." People tend to prefer things that belong to them, even over *identical* items that do not belong to them. Thus, you can be expected to like what you own.
3. "Self-enhancement." People tend to find ways to view themselves in the most favorable light. You're not the kind of bozo who would drop a ton of coin on a marginal improvement (or step backward) for your system, are you?
4. Self interest. No fancy name needed here. Dealers and manufacturers have a strong interest in promoting the "break in effect"; it makes a good answer for the disappointed customer who might otherwise want a return, and buys time for the psychological processes noted in 1-3 to do their work. Try calling a dealer and saying the item you just bought underwhelms you, and then ask about break in. Do you think the dealer is likely to say "break in is a myth"?
This is not to say that break in is never a factor, and still less to say that people cannot properly appreciate big, and even small, differences in gear. (Bring those big Rockports or Wilsons over, and I'll likely prefer them to my more modest speakers, ownership be damned.)
It is to say that the psychological evidence suggests people would experience the "break in effect" *even if there were no objective improvements in gear over time*. So I would like to see very compelling evidence before attributing the experienced improvement to the gear rather than the listener. (Note that the experience itself cannot be such evidence; it's not the experience that is at issue, but its source.)
John