Many thanks for the replies. So if I understand correctly, a well-mixed Dolby stereo track can be just as good and immersive as a 5.1 on the same setup?

How does the Pro Logic II know which parts to output to each speaker? I understand how it determines the low frequency stuff for LFE, but how does it know to put the dialogue in the center and what to send to the surrounds if the audio file itself is only stored as a 2.0 track and not the 4.0 that it was mixed at? I read about the Dolby Stereo Matrix on wikipedia, but I'm still not quite sure how this could provide as immersive of a result as a true 5.1.

Using Temple of Doom again as an example, is there a reason one would choose to play the Dolby Stereo track over the 5.1 DTS-HD on the Blu-Ray in a 5.1 setup?