burn in exists but it depends on the headphone and the person listening at the same time. Some headphones change more than others over time and sometimes even those cans that do change more over time may not have changed in the view of certain individuals that already liked them in the first place. I've noticed that crappy cans or headphones that people don't like seem to change more over time when it is the psychological factor that we are dealing with.

With experience and a separate headphone or speakers used for reference purposes, it is easier to tell whether a certain headphone has changed in sound over time.

According to some, the DT990, DT880, AKG701, Grados, among others benefit from burn in. Of all of those, I used the DT990 for a couple of months. They were too bright in the beginning. However, if I had them on for a few hours, their sound would perceptively improve (not to my complete satisfaction though). Furthermore, if I switched headphones and then put the DT990 on again, they would be very bright again. I liked them much more when equalized.

It may be me, but is it the trend that relatively brighter (or bass lean) headphones benefit more from burn in than other headphones? It is my understanding that direction can be better perceived with high frequency sounds. Regarding our mind "hand" on the matter. Could it be that our brain adjusts to high frequency deviations more readily given their importance in direction finding? Are there particular frequencies bumps that are more readily compensated by the brain?

As Purrin and MF said, the fact that burn in always results in a positive changed makes it highly suspect in my eyes.  But not only does it always result in positive changes, burn in causes all kinds of wildly different changes, that always happen to be the exact changes that headphone needs.  If a headphone is too bright, burn in tones them down, if a headphone is shrill, it becomes less shrill, if a headphone is basslight, the bass increases, if a headphone has too much bass, the bass lessens, if a headphone has boomy bass, the bass becomes less boomy, if a headphone lacks warmth, it gains warmth, if a headphone has small soundstage, the soundstage increases. 

Not once have I ever read about someone experiencing negative changes with burn in.

This just isn't possible with a mechanical change that isn't recognized by manufacturers, and therefore not planned or accounted for.  There's no way it could be that consistently positive while changing every single aspect of a headphone's sound in every single direction. And even if it was a real mechanical change, and was accounted for and planned for, look at the way most headphones are designed- most of them are horribly engineered seemingly by baboons.  The idea that these terribly engineered headphones always have perfectly engineered burn in characteristics is absurd. 

I believe the vast majority of burn in changes are in the brain.  But I do think it's possible that there is something that changes, sometimes, with some phones.  I don't think it's significant enough to worry about though.  I don't think someone should talk about burning in phones they don't like at first to see if they change.  I think it's likely that people's first impressions about a phone are more accurate, and the burn in effect is them getting used to those problems/colorations.  As has been said by others above.

Some interesting shades to this discussion so far. Good points brought up, but some false assumptions by the OP on his audience on this board.

Does Burn-In Exist?
Pretty sure no one here has completely closed the door on the possibility burn-in existing. There are several subjective accounts in this very thread where a listener has experienced headphones sounding differently in different instances and/or over time. However, given the complex interactions that factor into our auditory perception, it's hard to conclusively attribute the subjective differences to only driver burn-in. As you know, it is much harder to disprove something than to prove that something exists. And even then, there is no solid evidence that burn-in truly does exist.

Do measurements tell the whole story?
Just because the instruments we have on hand cannot prove burn-in exists in a meaningful way, that doesn't necessarily end the debate for me. I don't think anyone here will tell you that objective measurements (in its current state) is the end-all-be-all. I've seen Purrin and Anaxilus argue with hordes of objectivists that believe that the Objective2 amp is perfect because it measures so. Purrin himself has admitted that he can hear clear differences between different tubes although it does not show anywhere on his measurements here. While measurements give us an insight into what we hear, there are things that we must subjectively find out for ourselves (ie. resolution, or soundstage).

Do measurements create a bias?
Of course it can (and does). But I believe some people are more susceptible to it than others. I'm confident that most of the folks that frequent this site are intelligent to account for confirmation bias, and strive to be more honest with themselves. Folks here are definitely a far cry from the typical audiophile that slaps an expensive cable onto their headphones that instantly transforms it. And if you look through individual headphone measurement threads, there are subjective impressions from people who hear things a bit differently than the measurements show.

So my personal belief? Could be driver burn-in. More likely it is brain burn-in. Somewhere between the two likelihoods is the possibility that it is an interaction between driver and brain burn-in (and possibly others). But it is not a big enough deal to me that I spend too much time caring about it. When I buy headphones, I use them for 200 hours or so and then decide whether to keep it.