From Tyll's website :
http://www.innerfidelity.com/content/evidence-headphone-break
Here's a different, newer one, also with good info:
http://www.innerfidelity.com/content/measurement-and-audibility-headphone-breakI think there's no doubt headphones can burn-in, but to what extent a headphone will burn-in varies from headphone to headphone. I think most headphones lie in the territory where any changes from mechanical burn-in are likely to be so slight that they're not really audible (or, at the very least, not nearly as audible as most people claim, even for those headphones that nearly everyone say require hundreds of hours of burn-in, like the K701 or similar).
That said, I've given up really worrying about headphone burn-in. There are too many other factors that play into my perception of a headphone's sound at any given moment to make mechanical burn-in really that important. Plus, if I don't like a headphone out of the box, there's practically no chance mechanical burn-in will be enough to change my mind from "dislike" to "like."
Solderdude also touched on something I've brought up once or twice. I do think it's possible a headphone's performance could change over time as it adjusts to the climate/environment of your residency (consider humidity, temperatures, pressure, changes in elevation, etc.). In particular, if it's extremely cold out, and the headphones arrive nearly frozen, they might literally need to warm up. This is certainly true of some ear pads, which don't deliver optimal performance until they've warmed up on your head.
