Good article by Carolyn Costa on ESPN yesterday about Arroyo's struggles.
http://espn.go.com/blog/statsinfo/po...yo-this-season
The primary explanation: he's not locating well, especially when he's ahead in the count and should be putting guys away. He's forcing guys to swing and then giving them meatballs...
Here's an image showing where Arroyo is throwing his fastballs & changeups (2/3 of his pitches) in 2011 (left) vs. 2010 (right).