Plunks that could have been walks
So far this season 930 batters have been hit by a pitch. 188 of them were hit by the first pitch of their plate appearance, but only 50 have been hit after already taking three balls. 20.2% of plunks occur on 0-0 counts, but there are a lot of 0-0 counts - 109,456 of them so through July 23rd this year. Pitches thrown on 0-0 counts have hit batters 0.17% of the time, while for some reason 0.24% of pitches thrown on 3 ball counts have hit the batter. And, since there have been 930 HBPs this year, on a total of 419,450 pitches, that means 0.22% (CORRECTION BELOW) of all pitches hit the batter they're thrown toward, but they hit the batter slightly more often when the guy already has 3 balls in the count. That is not exactly what you'd expect to find if you worked under the assumption that batters have more incentive to get out of the way when the pitch that's going to hit them is going to otherwise be ball 4. I can only assume this means batters have adopted the HBP as a way to prove their grittiness to the opposing pitcher, and they don't care if they're getting hit by ball 4 if it gets their message across. I like that theory better than the idea that maybe pitchers make more mistakes under the pressure of a 3 ball count, or the argument that it could just be a fluke of the data this year, and it's not that significant a difference.
50 batters have been hit on 3 ball counts this year, but only Josh Willingham has done it twice - both times on 3-2 counts. Last year Chris Iannetta failed to avoid getting plunked 3 times on 3 ball counts.
UPDATE 7/27/09: Turns out I had a problem in my original query to determine the number of pitches thrown on 3 ball counts, which foolishly excluded pitches that were the 4th ball. So, the real answer as of today is that only 0.13% of pitches thrown on 3 ball counts hit batters this season, which makes a whole lot more sense. I've fired my proofreader and fact checker, but they've obtained a court order forcing me to allow them to keep their jobs, sighting discrimination, which doesn't make any sense since they're all just me.