Hmm. I think
maybe they changed the policy and now you can you can, but not with enough certainty to advise you to do so.
I am certain that when our season started it was prohibited, and that tablets appeared on the list of prohibited items in the same section as computers. But now that bullet point just reads "laptop computers," and the separate bullet point for televisions says "television sets, excluding television substitutes such as tablets" are prohibited.
http://mlb.mlb.com/nyy/ballpark/information/index.jsp?content=entry
Which is a bit confusing because tablets are excluded from the excluded items list, and it's got that whole double negative thing going on.
If you google "yankee stadium ipad" you will find dozens of articles advising you cannot bring them. There is one ESPN blog entry saying the policy changed to allow them in 2012,
http://espn.go.com/blog/new-york/yankees/post/_/id/34172/ipads-now-allowed-at-yankee-stadium but plenty of others since then saying ipads are forbidden. And I cannot find one in 2015 announcing they are permitted.
And just to confuse things even more the NYCFC App has a section on prohibited items and the bullet point for televisions does not have the clause excluding tablets from the exclusion, which means you cannot even show the security person if they give you trouble.