This wiki site has some info on it, and refers to an interview for a 1983 book, which I have not read, so I cannot speak for it's validity:
http://starwars.wikia.com/wiki/Sequel_trilogy
It links to an article that has quotes from the book:
http://www.epinions.com/content_4057571460?sb=1
Reading some quotes from it, there is little direct quote from Lucas, but it is offered up from the person who had interviewed him:
”If Lucas ever decides to take the story further, he will start by going back to a time before Star Wars began. He will then make a sequel to the Star Wars trilogy. Lucas can describe the stories, the plots, and who does what to whom in the three movies he calls the “prequel” to Star Wars..
(The “prequel” she refers to is the trilogy that (now) includes the three films: THE PHANTOM MENACE, THE ATTACK OF THE CLONES, and REVENGE OF THE SITH.)
Ms. Worrell continues: ”But he has only a vague notion of what will happen in the three films of the sequel…In the sequel Luke would be a sixty-year-old Jedi knight. Han Solo and Leia would be together…The sequel focuses mainly on Luke, and Lucas says Mark Hamill will have first crack at the part if he is old enough. ‘If the first trilogy is social and political and talks about how society evolves, ‘ Lucas says,’Star Wars is more about personal growth and self realization, and the third deals with moral and philosophical problems.. The sequel is about Jedi knighthood, justice, confrontation, and passing on what you have learned.'”
This is likely where a lot of the speculation had come from. I know that he walked that talk back a few years ago when the prequils had been released, trying to take away the thought of the "third" trilogy.