'Between you & I' is indeed a solecism, since 'between, as a preposition, takes the accusative. But there is, alas, respectable Bardic precedent for it: Antonio's letter to Bassanio, Merchant of Venice III ii, "... all debts are cleared between you and I if I might but see you at my death." It must be all right if it's Shax, as they say ~~ but this is till a usage I avoid like the plague!