Chris Dial, who does a defensive metric called "Defensive Runs Saved," has posted the stats for this season through August 8 at Baseball Think Factory. Like all defensive stats, DRS has its strengths and weaknesses, but one of the good things is that it elegantly summarizes a bunch of factors in one simple metric--the number of runs that the fielder prevented compared to average.
Chris's essay on the NL data and a commentary is here, and the data itself is here.
Probably most notable here is that Milledge has only cost the Nationals 1.6 runs in CF. That's better than most other metrics make him look, and if he's this close to average, he's definitely doing well enough to keep him there, at least in my book. Again, that's assuming his offense doesn't improve enough to make him a winning corner OF.
Other notables are Willie Harris, who is awesome, and Kearns, who is good, and Belliard, who is terrible. No suprises there.
Zimmerman doesn't look as good by this metric as you'd think, even considering the lost playing time. Guzman comes out right around average, which again isn't what I would have expected.
So take it FWIW.