Apparently George Lucas has an idea of another Star Wars movie - and it would be a prequel to Episode I. A scooper for Cinescape related to them his tale (use this link to read it and some other movie rumors), he apparently met George Lucas in Hawaii recently and asked him a few questions. Most notably, "I asked him if STAR WARS was going to end with this next installment and He said, "'Star Wars' will never end. My involvement may, but the story will go on....""
Then the scooper (credited in the article as MK) asked, "What do you think would be more interesting.... telling a story prior to PHANTOM MENACE or after RETURN OF THE JEDI?"
To which, Lucas replied, "No question about it, prior to PHANTOM MENACE." He said, that if he did ever do another storyline.... that he would do one of when the Jedi regained control of the universe from the Dark Lords (there were many of them) 800 years before PHANTOM MENACE. And a young Jedi named Yoda was instrumental in the effort.
Now I think it goes without saying that this is a rumor, it isn't fact - and there's no real way to confirm it (unless Lucas come out publicly and says it's true). Personally I think it's not true, and that George Lucas is personally done with Star Wars movies. He has talked publicly about the animated Star Wars : Clone Wars TV series, and another live action Star Wars TV series in development that he will have some involvement with. He's also working on Indiana Jones IV (he mentions that in the last article I linked). So it looks like Lucas will be keeping pretty busy - he probably wouldn't even have time to think about another Star Wars movie for a few years at least. So it sounds like just another internet-fueled rumor to me.