I need to break up Matthew's little hot streak. Whose blog does he think this is?
Running down the table:
-Ichiro's swing rate on balls outside of the zone hasn't changed at all
-Ichiro's swing rate on balls inside the zone is a little down, but probably not significantly so
-Ichiro's made more contact on balls outside of the zone
-Ichiro's made just as much contact on balls inside the zone as always
-Ichiro's groundball rate is up
-Ichiro's line drive rate is down, but probably not significantly so
-Ichiro isn't lacking for infield hits
-More of Ichiro's balls in play are being turned into outs than ever before
The first thing you might guess is that hitting grounders at the expense of line drives is hurting Ichiro, but I think the opposite is true - this should be helping him. In his groin-grabbingly awesome 2004, he put up a GB% of 63.7 and a LD% of 18.4. Fly balls are what he really wants to avoid, and he's been doing that just fine. So this likely isn't the issue.
The next thing you might guess is that he's being hurt somehow by making more contact on balls outside of the zone, but (A) that seems weird, (B) his O-Contact% was 77.8% last year and he did just fine, and (C) his BABIP on balls outside of the zone is actually higher in 2008 than in 2007, at least according to Josh Kalk's combined PITCHf/x tool. The culprit here appears to be BABIP on strikes.
Another thing I've seen here and there every so often is the suggestion that opposing defenses are learning how to play Ichiro better. They're not. This is, has always been, and will continue to be stupid.
You could go through a laundry list of potential reasons why Ichiro's numbers are down, but just because he's a unique player doesn't mean there has to be a unique explanation. He's been unlucky. Despite zero convincing evidence that his skills are eroding, he's struggling to match his recent batting lines, and when you see that happen to someone luck should automatically be the first thing that comes to mind.
Adjust Ichiro's performance so far to that .353 '05-'07 BABIP and you've got yourself a .319 BA with a .377 OBP. In other words, business as usual. Barring something unforeseen, I expect Ichiro's numbers from this point forward to look a lot more familiar than they have to date. If his batting average between tomorrow and the end of the season isn't in or around the .310-.330 range, I'll eat a hat.*