Is a card that was played and immediately trashed "in play"?
It is not considered in play if it has moved from your play zone to the trash.

The 'no more than one action card in play' trigger will not count mining villages that were played and then immediately trashed.

Sources: forum post by Donald X. (games creator)

http://boardgamegeek.com/thread/682203/description-sheet


  Walled Village: When you play this, you draw a card and can play two
  more Actions this turn. At the start of your Clean-up phase, before
  discarding anything and before drawing for next turn, if you have a
  Walled Village in play and no more than two Action cards in play
  (counting the Walled Village), you may put the Walled Village on top
  of your deck. If the only cards you have in play are two Walled
  Villages, you may put either or both of them on top of your deck.
  Walled Village has to be in play to be put on top of your deck. Walled
  Village only checks how many Action cards are in play when its ability
  resolves; Action cards you played earlier this turn but which are no
  longer in play (such as Feast from Dominion) are not counted, while
  Action cards still in play from previous turns (duration cards from
  Dominion: Seaside) are counted, as are Action cards that are in play
  now but may leave play after resolving Walled Village (such as
  Treasury from Dominion: Seaside).


The example 'Feast' in the quote is an action with a self trashing ability similar to mining village.