As seems to be the growing pattern, I can never keep my responses to other people's blogs down to under 750 characters...so, go read The Dark Moose's entry on this subject here, then come on back...

I guess my point of view on the nature of balance in the PT is this:

Since the Force affects and is affected by all living things, then it's not even just the Force-sensitives that have an impact on it's balance. The growing corruption of the Republic, arguably having begun before Sidious started working his machinations, and the complacency of the Jedi within this spreading corruption, in my humble opinion, is what brings the Force out of balance.

If you look at Yoda's comments in the novel version of ROTS, he claims to have failed in more than just his duel with Sidious:

"My failure, this was. Failed the Jedi, I did."

He spoke to the Force.

And the Force answered him. Do not blame yourself, my old friend.

As it sometimes had these past thirteen years, when the Force spoke to him, it spoke in the voice of Qui-Gon Jinn,

"Too old I was," Yoda said. "Too rigid. Too arrogant to see that the old way is not the only way. These Jedi, 1 trained to become the Jedi who had trained me, long centuries ago-but those ancient Jedi, of a different time they were. Changed, has the galaxy. Changed, the Order did not-because let it change, I did not."

Yoda is saying here that he was so focused on re-fighting the previous Sith war (and no, this is not an original comment, but for the life of me, I can't remember where I first read it...), that he was unable or unwilling to adapt to the times. The Sith, however, in their lust for greater power, constantly evolved, adapted, and eventually understood the modern world so well that they were able to orchestrate their elaborate, decades-long plan for obtaining total control over everything by manipulating both the Senate and the Jedi.

Having said all of this, it's my take that the imbalance in the Force came not necessarily from the Sith's rise to power, but rather from the bloated nature of the Republic, and the Jedi allowing their role in galactic affairs to stagnate.

I'm very interested to hear if anyone has anything to say on this, so please comment away!