was unmasked as industrialist and chemist, Norman Osborn.
John Romita later said:
Stan wouldn’t have been able to stand it if Ditko did the story and didn't reveal that the Green Goblin was Norman Osborn. I didn't know there was any doubt about Osborn being the Goblin. I didn't know that Ditko had just been setting Osborn up as a straw dog. I just accepted the fact that it was going to be Norman Osborn when we plotted it. I had been following the last couple of issues and didn't think there was really much mystery about it. Looking back, I doubt the Goblin's identity would have been revealed in Amazing #39 if Ditko had stayed on.
The rumor was that Steve Ditko wanted the Green Goblin to be a random mook, because in real life it's not always "the butler" who did it. Just like the recent resolution of the Crime Master story.
Later on came the rumor that Ditko was setting someone up near J. Jonah Jameson to be the Green Goblin, and a popular theory was Ned Leeds... oh, the irony on that one.
Recently, Steve Ditko came out and settled this rumor once and for all. In Robin Snyder's "THE COMICS" Vol. 20, No. 3 [March 2009] there's a two page essay written by Steve Ditko titled "The Ever Unwilling", in which he mentions the Green Goblin. Here are the relevant extracts:
"So certainly, the GG [Green Goblin] could hardly be any reason for me quitting Marvel"
"Now digest this: I knew from Day One, from the first GG story, who the GG would be... I planted him in J. Jonah Jameson's businessman's club." [prior issues of the newsletter had a contest to identify all the planted appearances of the character]
"I planted the GG's son (same distinct hair style) in the college issues..." [referring here to Harry Osborn]
So, after forty years, this urban legend has finally been laid to rest. Norman Osborn is the Green Goblin, always has been the Green Goblin and always will be the Green Goblin. He was conceived that way after all.
As for why Steve Ditko and Stan Lee really split up, I cannot even begin to speculate there. Except that I believe the answer is a lot more mundane and complicated than any urban legend could be. But even I always believed that a talented guy like Steve Ditko leaving Marvel over the secret identity of one character was something too absurd to be real.