I have to admit that I was surprised to read this, too:
Here's Dallas Braden after l'affaire A-Rod-walky-over-the-moundy:"The long and short of it is it's pretty much baseball etiquette. He should probably take a note from his captain over there, because you don't run across the pitcher's mound in between an inning or during the game. I was just dumbfounded that he would let that slip his mind."
And here was A-Rod's response:"He just told me to get off his mound. I was a little surprised. I've never quite heard that, especially from a guy that has a handful of wins in his career. I've never even heard of that in my career and I still don't know. I thought it was pretty funny, actually."
I wrote this morning that I had never heard of that particular unwritten rule. Since then, however, I've done a bit of Googling and read some stuff some other people have written and I think it's safe to say that it's at least a minor unwritten rule. It's not up there with "don't steal second when you have a 10-run lead" or "don't go one-flap-down on your home run trot unless you're Jeffrey Leonard," but it exists. It may be a dumb rule -- as so many of the unwritten rules are -- but it's a rule and ballplayers seem to care about such things.
Which causes A-Rod's comments to ring hollow. That guy has been around baseball his whole life, so I'm assuming he's heard of it. He may or may not have walked across the mound with the intention of getting under Dallas Braden's skin -- maybe it was just a brain lock -- but his response was truly intended to.
No, you don't "walk across the baseball mound." You don't tread upon the mound, period. That's not only his mound--Dallas Braden, the pitcher from the other team--but it is also his teammate's mound as well. And you don't walk on it. Period.