For the first time that I'm aware of, there is a state-of-the-art advanced fielding metric available to the public in nearly real time. Mitchel Lichtman's Ultimate Zone Rating will be updated weekly at Fangraphs.com.
Beware the very small sample sizes--you can't really evaluate any fielder based on one week's worth of opportunities--but you won't be surprised to learn that the Nationals are down in the bottom third in team defense. Click the link, and you'll see a table telling you that the Nationals have allowed 2.2 runs more than an average fielding team would have allowed given the same opportunities.
Looking at individual performances, Cristian Guzman has been our best, saving 0.9 runs above average. Zimmerman, Dukes, and Johnson have been above average as well, while Kearns has been exactly average in right. Our worst fielder has been Willie Harris, who has allowed a bit 1.6 more than average playing second-base--anywhere in the infield he's a below par fielder. Belliard's cost another run at the keystone, whille Alberto Gonzalez is 0.2 below average in his brief playing time. Willingham (-0.4) and Milledge (0.6) are both below average. Dunn's been terrible, costing us 0.6 in left and another 0.8 at first.
No big surprises there, but it'll be very interesting to watch over the course of the season how offensive and defensive contributions all add up.