Fenway has special seating and handheld assistive listening devices and
Gillette Stadium has great compliance with and surpasses accessibility recommendations & requirements. There's a very cool video accompanying the Gillette accessibility article.
Gillette Stadium: Accessible to All - YouTube
We've tried out a couple of the newer iPhone apps with ASL dictionaries targeted specifically at the installations at museums, one was being tested at the Museum of Science in Boston. It was a bit awkward to use with multiple, not yet interlinked dictionaries to walk through one by one and really didn't quite hit the right vocabulary, but I can see the potential for really making such a thing work.