Here's my Hardball Times column from this week:
On average, the difference between an average center fielder and an average left or right fielder is about 15 plays; that's about 10 to 12 runs, or approximately one win. Another way to think about that is that you gain one win above average by sticking a middle-of-the-pack CF defender (2006 So Taguchi, for instance, according to David Pinto's PMR) at a corner position for 150 games. On the flip side, you'd lose a win if you put an average corner guy (say, Luke Scott) in place of an average center fielder in center for 150 games.
It doesn't pertain too much to any current Brewers situations, but it'll be interesting to see how it pans out in Chicago if Alfonso Soriano becomes their center fielder.