My guess is that it is derived from version 1.10 of the OpenGL Shading Language, document revision number 59. Found it here, page 7: http://old.siggraph.org/publications/2006cn/course16/KhronosSpecs/ESLanguageSpec1.10.11.pdf The differences are also listed there with underlined text.
Can anyone confirm that it is based on version 1.10?
I took a look into the latest revision of OpenGL ES Shading Language - 1.0.17. It seems there is no reference to the original OpenGL Shading Language and no difference specs at all. The problem is that I can't find any document revisions between 1.10.11 and 1.0.17. They could change their mind and derive ES Shading Language from Shading Language 1.20. or something else.