:: Ok, I think I understand this issue now. First, this wiki-solution (which is found in various books) is horribly obfuscating because all probabilities there are conditional... However, in more clear sources , still using the canonical sample space {1,2,3}, part of the MHP problem assumption (host picks goat, host doesn't pick player's door) is an event', say A, so the question is to calculate the conditional probability of winning by switching, knowing that A happened, ergo the probability asked for is conditional on A, even though A'' happens with probability 1. However, I think there are alternative encodings, which account for symmetry, that don't have an explicit event encoding part of the problem. For example, the one in my sandbox. It seem unnatural to call the final probability there conditional. (It's probability space is constructable as multi-stage experiment, I think; the resulting sample space is bool x bool). Am I making any sense?    

 == Subjectivist interpretation == 
 By the way, the same source  has probably the most clear discussion of subjectivist vs. frequentist debate on MHP. I raised this latter point on the article's talk.