Can a unit that has already been hidden be played to the battlefield for its normal cost when Mageseeker Warden is on the battlefield?

Ruling: No. Once a unit has been hidden (placed face-down on the battlefield), it cannot be played for its normal cost. It can only be played for the hidden cost.

Nuances:

• Cards with the Hidden attribute can be played from hand for their normal cost instead of hiding them, but this does not apply to cards that have already been hidden.
• The distinction is between "a card with Hidden" (in hand) versus "a hidden card" (already face-down on battlefield).