“I’m trying to remember: was it H. G. Well’s The Time Machine where the protagonist wakes up clutching some memento from his dream?”
Actually, I heard a similar story in person, back when I was hanging out with Neo-Pagans. I’m sure the idea’s shown up in myth once or twice, and it certainly has in modern fiction.
And yeah, Gibson’s cyberspace is a direct steal from the traditions of the “astral plane”. This becomes more obvious with _Count_Zero_ and later books. (Vernor Vinge did it earlier and better in _True_Names_)