What it looks like:

    You flip the top card, revealing the Ace of Spades. You then give this card to the spectator to hold, face down. You flip the next top card, which is the Ace of Clubs, and announce: "I'm going to switch these cards."
  Your hands dart backwards and forwards, and you say: "Okay, I'm going to show you what I did, but in slow motion." So, you do it in slow motion, and quite obviously switch the cards. You then give back the Ace and say: "Okay, which one is the Ace of Spades, and which one is the Ace of Clubs?"
  The spectator answers, and you ask them to show the cards to everybody, and they are two queens. This trick has made many a jaw drop, and is responsible for me being chased down a corridor by a school mate, who wanted to know how on Earth that trick was possible.

How it's done:

1. Arrange the top 4 cards like this: Ace of Spades, Red Queen, Red Queen, Ace of Clubs.
2. Flip the top card, showing the Ace of Spades, and give it to the spectator to hold face down, with his fingers on top and thumb on the bottom.
3. Do a triple lift (pick up three cards to look like 1) and show the Ace of Clubs. You then put the card face down, and take the top card (which is a queen) and slide the card under the spectators fingers and out again. You are actually doing nothing when you do this, it just feels like you switched them.
4. Do it in slow motion, and this time, actually switch them. So they have a face down Queen and you have the face down Ace.
5. 'Give them back their card.' You are actually NOT giving them back the Ace, but the other queen. To do this, get eye contact with the spectator, then slide out the Queen from the top of the deck, put the Ace on top and take the Queen.
6. So then, after giving them the queen, ask: "Which one is the Ace of Spades, and which one is the Ace of Clubs?" After they answer, get them to flip the cards, and BAM - They have changed!!!

To see a video of this trick, go to http://www.youtube.com/watch?v=eZQ3BMukysg