Guy's I have noticed that on these forums people are skeptical about Burn-in with HD800 but I have to say that I have noticed a considerable difference between the music since I bought the HD800 in April. The sound has opened up considerably to a point where I am finding the experience more like listening to speakers. But initially the sound was more like a typical headphone. Its just become so enjoyable to a point where I dont miss speakers. Now I justify paying £1000 for a pair of headphones.
It is hard to say, but I believe that I do concur, with all the usual caveats re audio memory or your mind getting used to the sonic presentation from a can. I think I have had mine 1.5 to 2 years now.
I have heard people say that the treble gets toned down overtime as well. That is something I did not experience, but for me treble was never an issue.