My post below outlined the possible default rule and property rights issues involving Red Sox 1B Doug Mientkiewicz' decision to keep the ball from the final out of the World Series, rather than turning it over to the Red Sox.
Over at the Volokh Conspiracy blog, this comment by Todd Kincannon to Volokh's argument that the employer-employee relationship should govern the matter fleshes out an interesting distinction in the "custom rules" argument, which I had thought should prevail in favor of Mientkiewicz:
There is an extremely well established custom in Major League Baseball that players get to keep significant baseballs unless they end up in the stands. Umpires will stop games so that a player can retrieve a significant ball, such as the one he just got his 3000th hit with, or his very first hit, or was the ball he just made an unassisted triple play with, etc...
One potentially important distinction here is that this ball is not particularly symbolic of a personal achievement by Mientkiewicz (he gets a put-out on the play; not a huge statistical deal), but instead is symbolic of a Red Sox team victory.
Taking this argument to the next level, my friend Andrew "Hog" Warden has proposed a legal test that should be applied in the absence of an ex ante contractual clause. Although his sports acumen is sometimes clouded by his unwavering allegiance to anything Indianapolis-related, I am convinced this analysis hits the mark:
If I were a judge, I'd adopt the "significance of the ball test." First, you have to look at the significance of the ball and determine why that ball holds value. Here, the Red Sox won the Series, and not because of anything Mientkiewicz did. Second, you have to look at the claimant and determine his relationship to the ball's significance vis a vis other claimants. Here, no way Mientkiewicz prevails over the interests of the Red Sox. I suppose this test could fail at the margins in cases where multiple pitchers combine to throw a perfect game, but we'll save that analysis for another day.