I haven't heard of a fix coming. This is a significant problem, I own many non 5.1/7.1 blu-ray's that my AVR convincingly converts into 7.1 with DPLIIx. However, my guess is this isn't an easy fix for MS, otherwise I can't imagine that more people care about 3D than this issue (3D is coming next month).
MS is doing a good job adding a variety of channels to the X1 but if they can't playback the audio properly they aren't much use. Hopefully this gets fixed.