Russell and Norvig (1995) include machine learning as one of the four elements needed to pass the Turing Test. They cite Gold's Identification of Language in the Limit from 1967 as a laying the groundwork for machine learning applied to natural language processing.

http://stpk.cs.rtu.lv/sites/all/files/stpk/materiali/mi/arti...

http://web.mit.edu/~6.863/www/spring2009/readings/gold67limi...