WordNet (r) 3.0 (2006):

poker face
    n 1: a face without any interpretable expression (as that of a
         good poker player)

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "poker face":
   anesthesia, autism, catatonia, chill, chilliness, cold blood,
   cold heart, coldheartedness, coldness, coolness, deadpan,
   dispassion, dispassionateness, dullness, emotional deadness,
   emotionlessness, expressionlessness, frigidity, frostiness,
   heartlessness, iciness, immovability, impassibility, impassiveness,
   impassivity, inexcitability, inexpressiveness, lack of affect,
   lack of feeling, lack of touch, objectivity, obtuseness,
   passionlessness, self-absorption, soullessness, spiritlessness,
   straight face, unemotionalism, unexcitability, unexpressiveness,
   unfeeling, unfeelingness, unimpressibility, unimpressionableness,
   unpassionateness, unresponsiveness, unsusceptibility,
   unsympatheticness, untouchability, withdrawal