This thread is beating a long dead horse, but perhaps it's still an interesting discussion with some very valuable information. Let me just interject a couple of my observations/concerns:
1. I think there is certainly some strong credibility in the don't change the fluid in a non-maintained transmission, the question is, just what constituted a properly maintained transmission where one wouldn't be at risk, or at least significantly lesser risk, of dislodging sediment? IOW, what's the magic cut-off mileage or age? Regular 10,000 mile changes, 15,000 mile, 20,000 mile, yearly, etc?
I would guess mileage would be more important seeing as it is a sealed system. It is different than say engine oil. I think MM recommends changing "lifetime fill" ATF every 30k if you are going to do it. I probably wouldn't go much further than that if I were planning on changing it. I decided to leave mine alone just after 70k, which I felt was "higher milage" at over double his recommended interval.
2. Given that BMW specifically states lifetime fill on the transmission, one concern I might have is that if I did change the fluid, and something went wrong with the transmission, would they cover it under warranty since they specifically said not to mess with it?
Another good question. 
3. Not to add a 3rd component into the mix (engine oil, transmission fluid, and now...) but beginning 2005 model year BMW suddenly went to lifetime fill for the cooling system. Marketing decision? Does it fall under the same circumstances as transmission maintenance, etc?