UPDATE: June 27, 2012
I never did publish the Knock cards. I lack the skills to make them look like anything other than UNO card knock-offs.
The real reason for the update, though, is that I have found a much better service for printed playing cards:
I do not know if Guild of Blades is still in business, so I'll leave the rest of the post as is. In 2008, they were the best option available.
The fine folks at the Guild of Blades have opened up shop to print playing cards on demand. This isn't your traditional poker deck (although they can print those if you design your own face cards) printer, they focus on Collectible Card Games.
Andre Corvus, a story writer and game designer published this unsolicited testimonial of their services on YouTube. He used GOB to print beta versions of his card game, Renown. He talks about the quality and the differences from traditional cards. Overall, the quality and price are great - the cards even shuffle well.
I was happy to provide a blank template in the Yahoo! Group for their services, I'll let you know how good it is when I publish my own beta decks of Knock in May 2008.