You got it. The 6th channel is derived from the two surround channels. While there might be some issues with sound "bleeding" into the rear speaker from the other two surround channels, totally discrete separation is less of a concern in the back of the room, where sometimes it is better for the sound to "blend." That said, matrixing technology is no hard task these days, and the decoders are fairly sophisticated, especially when they only have to derive a rear surround channel from the two surround channels (whcih they do using phase-shifting). The thing that is really complex is de-matrixing a 2 channel, stereo signal into 7.1 discrete channels. going 2 to 8 channels makes going from 2 to 3 channels look like child's play.
Anyway - I also find it interesting that Lucas' loyalty to THX has limited him to Dolby technology, which is considerably more "lossy" than DTS, which his good pal Speilberg is a big supporter of. I guess it goes to show that profits and integrity are not always the best bed-mates.