re: It was LeGuin, in 1966
Card gives full credit to LeGuin for the use of the word. See here: http://www.hatrack.com/research/questions/q0004.shtml
The "ansible" is a longtime device in science fiction. In the broadest sense, many stories have needed to take place in a universe where travel is limited by lightspeed, but communication is not. Specifically, calling the communication device an "ansible" originated with Ursula K. LeGuin, or at least that's where I first encountered the word; if she had a source for it, I am unaware of it.
I used LeGuin's word for the same reason that we all use "robot" and "waldo" today -- when somebody defines a concept, even if we can never build it, we use their word for it (if it's a good word, as "ansible" is). That way writers get to live on in the language, even for people who never read their work. Thus Capek has "robot" even with people who've never heard of "R.U.R.," and Heinlein has "waldo" even for people who've never read "Waldo the Magnificent." LeGuin is entitled to ansible, even though I suspect she'd rather be remembered for other things <grin>.