Scientists say that poker may be tougher to crack than chess/checkers because of the uncertainties introduced by hidden cards.
http://www.nytimes.com/2007/07/26/business/26poker.html?_r=2&adxnnl=1&oref=slogin&adxnnlx=1185467840-t4htAYaCiLZLHBVvKhSRyQ&oref=slogin
Poker tougher to crack than chess/checkers