In the July 2009 issue of Baseball Digest, George Vass lamented the apparent disappearance of the "little guy" from Major League Baseball. Using my newly-found database skillz, here's a graphic comparing mean pitcher & hitter heights in MLB to average US male heights. MLB data are from the Baseball Databank, US data from here and here.
Average height certainly has increased over time, such that modern players would tower over players from the deadball era. But around 1960, if you believe the data, the US male population height seemed to stabilize, whereas baseball player height has continued to climb. This either reflects a problem with the data source(s), or, perhaps, an indication of the increased level of competition following integration and expansion of the player base into Latin American populations.
Another finding is that average height for pitchers has historically--all the way back to ~1890--been roughly an inch higher than that of position players. Not surprising or novel, but nice to see. Clearly this is not a new trend.