You got me there: I was going to write that "Apocalypse Now" was the first to use split surround channels (L-C-R-LS-RS plus one baby boom). That was in the information available at the time of its release. But I looked for a cite and found this on "Apocalypse Now"'s recording sessions:

http://mixonline.com/mag/audio_apocalypse_redux/
The eight-month (yes, this is also probably a record) re-recording schedule resulted in what can certainly be called the first 5.1-channel mix, in that it makes full use of stereo surrounds and low-frequency enhancement. (Although the Dolby 70mm 6-track “split surround” format was developed for Superman in 1978, that film played in less than a handful of such engagements worldwide.)

I don't understand, though, why they say AN "can certainly be called the first 5.1 channel mix" if in the next sentence they say "Superman" played in that format in at least a "handful" of engagements several months earlier. Must be some new definition of "first."

A sound engineer mentions that for "Superman" only two cinemas (one in L.A., one in London) were converted in time to play the new format:
http://hometheaterhifi.com/forum/archive/index.php/t-2897.html

David