Yes, it is a little off the beaten path for me to write a serious analysis about Star Wars. But I see the question so often that I felt compelled to lay out at least what I understand to be fact vs. fiction regarding Balance in the Force.

From the introductory documentary for A New Hope, Special Edition (on the VHS version), George Lucas discusses the (then) upcoming Episode II and III films, and how they tie in with the OT.

And the George spaketh unto us:

"The first film starts with the last age of the Republic, which is it's getting tired, it's old, it's getting corrupt.

There's the rise of the Sith, who are becoming a force, and in the backdrop of this we have Anakin Skywalker, a young boy who is destined to be a significant player in bringing balance back to the Force and to the Republic...

Then in the second film we get into more of that turmoil. It's the beginning of the Clone Wars, it's the beginning of the end of democracy in the Republic, sort of the beginning of the end of the Republic. And it's Anakin Skywalker beginning to deal with some of his more intense emotions of Anger, Hatred, Sense of Loss, Possesiveness, Jealousy, and the other things he has to cope with.

And then we will get to the 3rd film where he is seduced to the Dark Side..

Which brings us up to the films 4, 5, and 6, in which Anakin's offspring redeem him and allow him to fulfill the prophecy where he brings Balance to the Force by doing away with the Sith and getting rid of evil in the universe..."

Lo, verily didst he speak. So it is written, so it shall be...

With this information (this being only one of several sources and quotes on the matter) let's look at common misconceptions and their corresponding realities in George Lucas' intentions for the arc of the saga.

Fiction:
The Force is out of Balance because of the number of Jedi vs. the number of Sith. The Force is balanced when good is perfectly balanced with evil, and therefore any larger number on either side cause it to wobble right off of it's cosmic nightstand.

Fact:
George Lucas himself has said that it's merely the presence of the Sith that cause the Force to be out of Balance. It has nothing to do with the number of "light side" vs. "dark side" forces. The Force, in it's natural state, is balanced and harmonious. The Sith affect the Force so drastically that the Dark Side permeates its entire fabric. By destroying the Sith, Balance is restored. In fact, his exact words were "getting rid of evil..."

DM's Explanation:
Think of the Force as a river through a valley. It carves a natural path, and exists on its own accord, both serving and being served by the landscape around it. Think of the Jedi as tribe of aboriginal beings, indiginous to the landscape surrounding the Force. They draw on the waters of the Force for sustenance without disturbing its course. The Sith, on the other hand, you may consider as a Power company, come to the valley to harness the River of the Force for their own gain. They dam up the river, process its waters toward artificial ends, and upset the delicate ecological balance of the entire valley. As the River is darkened by the byproducts of the Sith, so is the world around it. Remove the Sith Power Plant, you allow the ecosystem to recover from it's effects.

Fiction:
Luke was really the Chosen One, not Anakin, because he destroyed the Emperor and Vader.

Fact:
George Lucas maintains that Anakin was always the Chosen One, redeemed by his compassion for his son, thereby destroying the Emperor, and himself. In doing so, he destroyed all of the Sith that existed, and restored the Galaxy from the Dark Side.

DM's Explanation:
Luke's role was complex. Though it was chiefly thought by Obi-Wan that he should destroy Vader, Yoda was more subtle in his training. Yoda only told Luke that his path to being a Jedi meant confronting Vader. Though Obi-Wan expressed disappointment that Luke can not kill his own father, which he doesn't, Yoda only offers the guidance Luke needs to bring him to that moment. Luke's role was never to kill Vader, nor the Emperor. It was to be a catalyst for the redemption of Anakin, to bring him away to the Dark Side so that he could finally fulfill his destiny as the Chosen One.

One could even say that for Anakin to have been the Chosen One, he had to fall to the Dark Side. Who better to kill an all-powerful tyrant than the person closest to his right hand? Regardless, the Prophecy's providence was not so clearly detoured - it may be that the Prophecy was fullfilled in exactly the way it was meant to. Anakin would sacrifice himself, and his soul, to be the Chosen One. It could be said, at least. Dunno - that's up to you.

Fiction:
The Force was not out of Balance until Palpatine turned Vader, and erected the Empire.

Fact:
The Force became out of Balance the moment the Sith had resurfaced, presumably shortly before the events of The Phantom Menace.

DM's Explanation:
Though this exact moment is unclear, it is undeniable that from the first unsettling events in The Phantom Menace, the shroud of the Dark Side begins to fall, and the Force begins to slip out of Balance. When the Sith reveal themselves to the Trade Federation and set off the alarming chain of events that brought the first open warfare to the Galaxy in an era, their effects became clear. The presence of the scheming Sith, just as Mr. Lucas says, is all that is needed to tip the scales of fate.

The Force is an elusive idea in the Star Wars universe, and is very nearly its own character with its own volitions. I often wonder about the idea of the "will of the Force", that being its ability to forsee what would happen, and devise its own solution in the form of Anakin. Sort of a self-correcting system of life and harmony.

Dunno. I'm just a moose. I hope that, my opinions aside, this helps to clarify some oft-asked questions about George Lucas' intentions when discussing Balance in the Force.

DM out