Interesting (to me, at least) observation:
I've run the video test files from here:
and the m2ts files from here:
and ran through the latest firmwares available for my (pre C+ [unfortunately]) PBO. I noticed that a few files showed up as "stereo," which I though was odd for demo/test material.
I turns out the files are PCM, and only the MedeBO firmware plays them as 5.1; PBO and ACR play 2.0 stereo.
From Steve's disc, the files are:
03.m2ts Cars (visuals)
20.m2ts Pearl Harbor (LFE)
38.m2ts Black Hawk Down (surround)