Poke around the net enough and you can find anything. Got this link http://www.cardsorcerer.com/markedcards.htm at a forum along with a clean joke.

Player A: I knew you were cheating!
Player B: How'd you know that!
Player A: Because those weren't the cards I dealt ya!

Interesting site with cards and literature for sale. Says you can use these cards for "stunning magic tricks" and a large scope of "effects". Here's another story from a thread at .rgp about marked cards at a Reno casino.

Wife: I don't understand why my husband wins so much money at cards but loses so much at the track.

Friend: That's easy to explain. They don't allow him to shuffle the ponies.