There are several different ways to consider the ontological reality of probabilities (i guess wikipedia only has two listed, Frequentists and Bayesians http://en.wikipedia.org/wiki/Probability_interpretations ).
The thought experiment in question probably should be answered with a Bayesian world view, and does not appear to be.
Prisoner A's chances of being executed is not materially effected when he learns new information, because the decision as to who has been pardoned has already been made.
Finding out new information, does change a probability, but not the probability that he'll be executed. It only changes the likelihood of whether or not the assertion that he will be executed is true. The nature of the universe has not actually changed, merely the state of belief of Prisoner A.
The point being, he's got no cause to celebrate, upon merely discovering information, because learning more information does not actually change the state of the world, in this case.