Originally Posted by Martian

Whatever they want to be called.

Sure, calling a senior individual by their first name when uninvited is disrespectful. When meeting a friend or girlfriend's parents, I always start out with Mr. or Mrs. (or Ms., if appropriate) X. If I'm uncorrected, that's what it stays. If, however, I'm asked to call them by their first name, I reckon it would be a greater disrespect to go against their wishes.

All in all, it's not generally that big a deal. Just do what comes naturally.