From a Kottke post:
"Mx." (pronounced "mix" or "mux") is a gender-neutral honorific. It's used by people who don't want to be identified by gender, whether their gender identity isn't well-represented by the older forms, or they just don't want to offer that information or assume it when addressing someone else.
This is happening. It's happening in progressive, diverse, digital communities first. And for all their fractiousness, and the inherent difficulty in dealing with areas as complex and personal as identity, gender, and sexuality, it does feel like some standards are emerging.