Cricket must be the last major bat/racquet type sport to only allow wooden hitting implements.
Baseball - MLB Rules state the following;
The bat shall be a smooth, round stick not more than 2.61 inches in diameter at the thickest part and not more than 42 inches in length. The bat shall be one piece of solid wood.other than that though, I don't think there is any more!
From what I've read Mythbusters disproved any improvement in performance when the bat was filled with cork? maybe this would be the same for cricket bats one day - who knows?