I went to a game at Safeco 2 years ago wearing a Yankees hat (although the Yankees weren't there) and there was no problem.

Honestly, you can safely wear anybody's paraphernalia in Yankee Stadium, except Boston's. We have to draw the line somewhere. Neither would I wear my hat in Fenway, unless I was suitably accompanied.

Thank you Yummyham (an Angeleno, no less!) for setting the record straight about New Yorkers.