Since Gardner, in his letter to Klarner dated April 16, 1990, says that a friend wrote to him about the two tilings problems a month or two earlier, and since Conway and Lagarias' article on the same subject came out that same year, I think we can safely rule out the possibility that Conway and/or Thurston learned about the problem from Gardner. (Alas; it would've made a nice story about Gardner's impact on mathematics.) Jim Propp