Jim Caple lays out some of sports' unwritten rules. It's a fun piece, though I have to say I preferred Rick Reilly's treatment of the subject a few years ago.
Only problem is, his first example, Pedro Marinez fraternizing with the opposition, is acutally a written rule.
Players in uniform shall not address or mingle with spectators, nor sit in the stands before, during, or after a game. No manager, coach or player shall address any spectator before or during a game. Players of opposing teams shall not fraternize at any time while in uniform.
Emphasis mine. Anyway, the rule isn't often enforced, but it is an actual writtern rule, not an "unwritten rule."