Great quote from Popov v. Hayashi (.pdf link), the lawsuit that determined ownership of Barry Bonds's 73rd home run ball:
The parties have agreed to a starting point for the legal analysis. Prior to the time the ball was hit, it was possessed and owned by Major League Baseball. At the time it was hit it became intentionally abandoned property.1515 See generally, Fugitive Baseballs and Abandoned Property: Who Owns the Home Run Ball?; Cardozo Law Review, May 2002, Paul Finkelman, (Chapman Distinguished Professor of Law).