Surround, it should be noted is an effects channel. It should not be confused with a four channel system intended to place sounds anywhere in the room. The standard placement of surround speakers is on the side walls at your seating position, not behind you. That is why they are called surround as opposed to the rear. There can be a center channel placed in the rear center and left and right rear speakers placed behind you as well in the 7.1 systems. These channels will most likely be matrixed as opposed to discrete. After all the movie producers want your attention primarily ccentered on the screen not what is sneaking up behind you.
Now it does seem that there are some DVDs that are encoded with a discrete center rear channel. I found a list at
http://www.dvdtherapy.com/dts_list.php, so it seems as though tvine2000 had a point in post #2. I'm not sure it was a life and death thing or anything like that, like the tone of that post seemed to be, but maybe tvine has spent too much time on other forums where the standard practice seems to be playing gotcha.