I have seen phototransistors with a connection to the base. The datasheets indicated it was to bias the transistor: make it more or less sensitive to light. I suspect the same is true for the 4N25. (By I could easily be wrong.)
Chris Savage agrees...
http://forums.parallax.com/showthread.php/118208-4n25-Base-pin-to-ground-or-not
More references...
http://www.technogumbo.com/projects/Learning-to-Use-an-Optocoupler-or-OptoIsolator/
What's odd is none of the half-dozen datasheets for 4N25 mention what to do with base. Where is the "typical applications" section?