Actually I was reading that the pitcher has to unambiguously declare which arm he is going to throw from. The glove in one hand, and the ball in the other when stepping onto the rubber is good enough. The batter then decides. One pitch has to be thrown from that side, and then the they each have one option to switch during the at bat. After that, they can't switch sides.
There are other caveats for outlying circumstances.
http://en.wikipedia.org/wiki/Pat_Ven..._Venditte_Rule