I just dipped into Cybertext by Espen Aarseth again, searching for the difference between Cyber- and Hyper-texts, and this is how I understood it:
Hyper-texts are the structuring of texts with links, whether it is mediated by paper or computers. Cyber-text is a perspective of texts where a text is not the same as the message in a sender-message-receiver-model, but a part of different communication systems where the system takes part in the creation.
Not clear yet? I'll be back to this, but I do feel that a hypertext is a series of links in a tree- or net- or what-ever structure, while a Cybertext is a game, where the text isn't even created until that stage in the game "happens." I am tucking the book into my bag right now, bringing it with me home to read - again.