It's become one of the rites of the Home Run Derby, and we'll see it again at the 2012 All-Star Game: Particularly die-hard fans are, at times, more worried about their player's health than they are excited that he was chosen for the derby--which I, at least, otherwise love to watch. But allow me to confirm this, for my fellow St. Louis Cardinals fans: Carlos Beltran will almost certainly hit fewer home runs after the All-Star Break than he did before. But it's not because of the derby.
It's because he only got to the derby because he was having one of his best first halves ever. Beltran was outstanding last year, and even then he's almost passed his 2011 home run total already. He's playing out of his mind, and the odds of him continuing to do so--at least, exactly in this configuration--are remote whether he participates in the Home Run Derby or not.
So don't worry about the Home Run Derby, because you have bigger problems to deal with: Namely, regression to the mean. It gets everybody.