I recently read this in a quasi-responsum online:
"Kilayim is specific to EY and areas that Hazal determined to have Q'dushas Ha'aretz. Outside of this zone, Kilayim is not applicable."
(Source: http://www.aishdas.org/avodah/vol17/v17n020.shtml)
Does anyone know which places other than Israel the author was referring to when he mentioned "areas that Hazal determined to have Q'dushas Ha'aretz"?