Although I suppose Polo and Forbes are big brands in modern times. I just doubt they'd put it on a baseball park. SCEA's released this video of the six classic ballfields available with an MLB 10 The Show pre-order.
The Polo Grounds, Forbes Field and four other fields will be unlockable from within the game, but you will have to play a lot to get all six, I understand. It's likely that at some point this will be made available as DLC to the general population but specifics on that are unknown. I'd say probably in the second half of the MLB season, to protect those who ponied up in advance.
MLB 10 The Show is available March 2.