Very interesting newsbit on MLB.com:
Batter's eye: Former Major League outfielder Darryl Hamilton, who works for Major League Baseball on-field operations, was in St. Louis on Wednesday. Hamilton was checking into complaints over the batter's eye in center field at Busch Stadium. Instead of a solid dark background, hitters are forced to look at dark blinds through which an Edward Jones advertisement can be seen.
It has been there since the opening of the stadium (if I recall), so I'm not sure what effect it really has had on the games.