Have the rules changed?

And if so, why didn't someone tell me?  There is a line in the play in which I currently have a small part (not my line) in which one character says to the other "Let's keep this between you and I." A thousand or so years ago when I was in elementary school, we were taught that if the pronoun was the object of a preposition - in this case 'between' - it should be 'me' not 'I'.  It's perfectly ok to use 'you and I' as a subject as in "You and I need to stop obsessing about this." but the sentence in the play should be "Let's keep this between you and me."  Or, if you want to keep things really simple how about "Let's keep this between us."?  In case you're wondering, I did point this out to the director (nicely, I think) and she said she would mention it to the author, but nothing was changed, so that;s why I'm wondering if the rules have changed.  If so, I wish someone would let me know.  Of course I could google grammar rules (and when did google become a verb?) and I will, one of these days. Or maybe I'll just keep thinking that I'm right and everyone else is wrong.