Nothing in particular. I just say hello! When I'm talking about Paul's mother, I usually call her my Mom-in-law. If I call her, I say "Hi, it's Karen!" Never really solved that one, I guess!

You can refer to them, for now, as Mr. Xyz and Mrs. Xyz, and if that bothers them as too formal, the impetus is on them to say, "Oh, dear, you can call me 'whatever their choice' is.