Feeling Cheeky, but have a probability question...

I would say the following (a physicists perspective maybe):
Saying something cannot happen and assigning a probability of zero to that event are exactly identical. One might argue that it is pointless to do so but such a zero probability can have some merit. For example: In quantum mechanics you will express the probability for a system for going from state a to state b, which would read (in qm notation): P(a->b)=C*< a|H'|b >
where basically a and b are the functions describing states a and b and H' describes the way in which the system is being influenced. The construct <|> is just a short notation for a certain mathematical operation (taking the complex conjugate of a multiplied by the operator multiplied by b and integrating over R3 usually).
Such a probability can turn out to be exactly zero for two given states a and b (not approximately but exactly). This then basically tells you that your system will not change from a to b (or vice versa) for the stimulus provided (which is described by H'). So a probability of zero can make a lot of sense...