WhisperSystems have updated their IOS app Signal so that it now supports private and secure text messaging between IOS and Android devices using TextSecure (the Android equivalent of Signal):

We cannot hear your conversations or see your messages, and no one else can either. Everything in Signal is always end-to-end encrypted, and painstakingly engineered in order to keep your communication safe.

The EFF agrees - no ads, no cost, no catches. A great way to claw back some of the privacy ground ceded in the name of convenience.