The ruling on this one is actually very surprising. I checked with the local MTG competition judge last night, as I too have seen various rulings.
P/T is checked in the very last layer, AFTER all the other effects. So it you have slagwurm armor on (say) a 2/2, make it unblockable as P < than 2, and then switch P/T it will be an 8/2 for damage purposes.
Likewise, a 1/5 enchantment will give a creature 5/1 when P/T are switched until end of turn. Even more surprising, instants do the same thing even when applied after P/T has been switched until the end of turn. So, switch a creature's P/T until end of turn, attack, then use an instant that should give it +0/3 and it gets +3/0.