Effect:

A spectator signs a card, the card is placed in the deck, and the cards are all shuffled. The Magician throws the cards at a bottle behind the spectators. The one signed card appears in the bottle.

Secret:

Beforehand, David blaine showed the spectators a different card trick involving the same spectator signing a card of the same number and suit that is forced when showing the card in the bottle trick following after the card trick. He then keeps the card and does another trick not involving cards, and slips the signed card to one of his camera men or lights crew. The guy then puts the card in the bottle. While David distracts the audence with the trick. Then he goes on to perform Card in the Bottle, and the card is forced, signed, and the trick is done. The magician throws the cards at the bottle, and holds on to the card that was JUST signed, so when he tells someone to look at the bottle, everyone is distracted, and he slips the card into his pocket.