Check this page out:
http://www.headwize.com/tech/dolby1_tech.htm
"Dolby analog movies and Dolby Surround video and television programs ..... are made using a Dolby MP (Motion Picture) Matrix encoder, which encodes four channels of audio into a standard two-channel format, suitable for recording and transmission in the same manner as regular stereo programs."
"In the theatre, a professional decoder is part of the Dolby Stereo cinema processor used to play 35mm stereo optical prints. The decoder recovers the left, center, and right signals for playback over three front speakers, and extracts the surround signal for distribution over an array of speakers wrapped around the sides and back of the theatre. (These same speakers may also be driven from four discrete tracks on 70mm Dolby Stereo magnetic prints, but in this case no decoder is needed.)"
"One of the original goals of the MP Matrix was to enable Dolby Stereo soundtracks to be successfully played in theatres equipped for mono or two-channel stereo sound. ..... Since the three front channels of the MP Matrix are assembled in virtually the same way as a conventional stereo mix - left in left, center equally in left and right, and right in right - playing a Dolby Stereo mix over two speakers reproduces the entire encoded soundtrack. There is only one thing missing: the surround signal is not reproduced in its proper spatial perspective. When the first home decoder was developed in 1982, its goal was to recover this missing spatial dimension."
Dolby Surround (the 1982 development) was devised to emulate the effect of Dolby Stereo in a home environment by recovering the extra surround sound effects.
I've seen DVDs in stores [case in point: Chaplin's Essanay Comedies Vol. 1-3 released by Image Entertainment] that state 2.0 "Dolby Stereo" soundtracks listed on the back of the case. Would it be possible to simply run the 4-track master through a new unlimited Lt-Rt Dolby Stereo soundtrack master like they did for the laserdiscs (without the "sweetening") and tout it as an actual 2-track Dolby Stereo mix on the back case of the DVD? It doesn't necessarily have to be "limited", although they could still do that if they wanted to. Just a thought.