[Chris J]

Or just put your headphones on your head and enjoy your music!

Forget about all this "burn-in" foolishness...

+1
I've never heard the effects of burn-in on any 'phone that I've owned.

 

[pp312]

  I never understand the argument that burn in is just your ears getting used to it because that's only taking into account the people who wear their headphones through burn in. I always burn them in with pink noise from my sansa clip and just throw them in a drawer with the clip connected to a portable battery for a few days. With some headphones, a major difference can be observed, with others the most you might get is improved low end extension. You never really know what you are gonna get but in my experience, sound does change at least a bit.

 
 
Pretty much sums it up. It isn't your ears getting used to the sound because so many people, myself included, never wear the phones during burn in. Also some phones exhibit considerable improvement, some hardly any and some not at all, which wouldn't be the case if we were all just getting used to the sound. So scratch that theory.  
 
But really, is burn in such a way out phenomenon? Why do car engines need to wear in? Why do we all tend to stretch when we get out of bed in the morning? Everything needs to limber up and get to optimum operating condition. I just don't see any great mystery here.

 

[drader]

Take a piece of mylar and bend it back and forth a billion times; it should end up slightly more flexible. Now take your brand new earpads and squish them to your sweaty ears  for a few hours; they will squish into position, and probably end up a few millimeters closer to your ear. Now run some current thru all that copper; be sure to analyze the alignment of the atomic structure before and after. I have actually done a test with a control headphone and a burn in one. The short answer is "yes", and anyone would be able to hear the differences. Some have a more dramatic puberty than others.

 

[tinyman392]

   
+1 
 
[excepting the part about cables - not sure about those yet...)

 
As you use a cable, especially a copper one, it tends to oxidize with time.  This can been easily seen in clear/silver colored IEM cables.  As you use them, they turn green.  This green layering isn't conductive, so this reduces the cross-sectional area of the cable.  When you reduce cross-sectional area, you increase resistance.  This is the equivelant of adding a resistor to your cable setup.  The size of the resistor depends on the thickness of your cabling and the thickness of the oxidized layer.  In general, the thicker the cable (metal portion only), the less difference it'll make.  
 
For IEMs that are extremely sensitive or have a very non-linear impedance curve, just about all BAs, lots of dynamics as well, this can make a difference.  But at the same time, it may not.  That said, the cable breaks in, I'll still question whether or not it's audible though.  

 

[tinyman392]

  Take a piece of mylar and bend it back and forth a billion times; it should end up slightly more flexible. Now take your brand new earpads and squish them to your sweaty ears  for a few hours; they will squish into position, and probably end up a few millimeters closer to your ear. Now run some current thru all that copper; be sure to analyze the alignment of the atomic structure before and after. I have actually done a test with a control headphone and a burn in one. The short answer is "yes", and anyone would be able to hear the differences. Some have a more dramatic puberty than others.

 
I think that as time goes, it becomes more than just a few millimeters  Pads flatten and warp quite a bit.  I know the pads on my HE-560 aren't anything like they originally were  Same thing happens to ear tips actually (silicone) for IEMs.  My Etymotic tips have warped into an ovular shape (I've got 3 sets like that as of right now)  
 
As for the everyone should be able to hear the differences, I can debate that one...  Everyone's sensitivity to sound, and change of sound, varies greatly.  Some may not be able to hear it over time.