'Back then, I thought, 'Well, there'll be other days.' I didn't realize that that was the only day." Thus spoke Dr. Archibald "Moonlight" Graham, or at least a whimsical version of him brought to life by Burt Lancaster in the 1989 baseball—some would say cornball—film "Field of Dreams." "One day, you're on top of the world. Next day, you've got to pick yourself up." So says Adam Greenberg, 25, a baseball player presently without a team, not yet sure if the one day he spent on a big-league field last year will turn out to be his only day. Greenberg...