"Also, MBJ... did you just take the raw digital audio from the THX LD's and pass it through a DD decoder to produce the 5.1? And if so, wouldn't this be essentially the same as the retail DVD's soundtrack passed through a DD in a 5.1 home setup?"

Actually, it was a software-Pro-Logic decoder, but correct otherwise, though I don't know anything about how the current DVDs made theirs, nor have I bothered to find out (this isn't aimed at you, BTW.)

And final soundtrack options haven't been entirely nailed down yet.