Just saw this thread again today. I have the Z5500's on my Mac Pro, but I only use MacOS X. When you say up-mix (2.1 --> 5.1) are you wondering if it sends stereo sound to all 5 speakers. The answer is yes. Ie. the 2 on the left play the left channel and the two on the right play the right channel. Not sure what the center one does, never checked.
Thats if you run it in standard 5 speaker mode. For example if i switch my Yamaha amp to run in Pro logic II mode then it will run the vocal through the center channel and the music though the other speakers. Of coarse its not perfect as its done from a 2 channel signal.