On thing to consider though is that this card's "in play state" is face up on top of your deck. It cannot trigger its response from anywhere else that I can tell.
A problem with it "being played" is that how do you mark if it has been played or not? The state is the same when it is put face up on top of your deck from the prism just as it would be when you played the action of the prophecy.
I still doubt it. If my opponent plays 'Eyes In A Jar' forcing me to play with a revealed hand, are the cards in my hand suddenly all considered to have been played and I'm free to trigger all of their abilities? I don't think so.
Regarding the marking: Well, if you cannot remember if your opponent played the Prophesy or not, I suppose, she'll be happy to put some kind of marker on it to remind you of it.
I'll postpone sending a rules question to Damon since it's still some time until the cards are published and I'm pretty sure we'll see another preview until then, and probably one focusing on new concepts like the Prophesy cards.