Is it Possible to Build Up an Immunity to Poison Like They Do in the Movies?

In one of my favorite movies, the Princess Bride, the hero Westley challenges Fizzini to a battle of wits.  Westley puts a deadly poison, iocane powder in one of two glasses and Fizzini has to choose which one to drink.  

Eventually Fizzini chooses and drinks his wine and then keels over dead.  It turns out that Westley had tricked him, he had put the iocane in both glasses, but Westley had build up a tolerance of iocane powder so that it wouldn't kill him.  So this raises the question:  Is it really possible to build a tolerance for poison?  Or is this just a crazy action movie plot point?

Well I don't know about iocane power specifically, but I have found out that you can build up a tolerance for another kind of poison, arsenic.  Arsenic is a toxin that binds to and inactivates proteins that are essential to metabolism.  However it can be inactivated in the body by enzymes called metallothionines, and the presence of arsenic can induce liver cells to produce more of these.  If small quantities of arsenic are consumed over a period of time, then enzyme production will be induced more often and background levels will increase, thus allowing you to survive a dose of arsenic that would normally be lethal.  

In fact there is even a word, mithridatism, which means the phenomenon of partial immunity to poison acquired by taking small doses.  Mithridatism was named after Mithridates the Great, King of Pontus from 120 B.C. to 63 B.C.  One of the legends of Mithridates VI was that he was worried that someone might try and poison someone of his stature so he created a universal antidote consisting of opium and honey.