I think I saw a similar speculation somewhere else in this monster thread before I bought a pair of TH900 for myself, so I paid a lot of attention to mine as they burned in and also did not listen to these 'phones exclusively during their first 100 hours or so. I do think that the sound signature changes noticeably in the 50-100 hour range; I've heard others say that the sound "opens up" and I'm not sure exactly what is meant by that (I'm not nearly as experienced in audio-lingo as many, and I'm certainly no sound engineer) but to me there seemed to be a better melding of the various instruments even as each one retained its discrete voice resulting in what I would call a fuller/richer/more euphonic sound. I felt that the bass and treble calmed down a bit resulting particularly in a very deep, tight bass that frankly blows away my LCD 2.2 and that I found preferable to my neighbor's HD 800.
(My listening to the TH900 has been done primarily on two systems: MacBook Pro or iMac>Violectric V800>V200; iPod Classic>Pure I20>RWA Bellina Pro HPA (Miniwatt Dario tube). The latter is my keeper: completely off the grid!