I talked with a guy who raised $220k. He saw about 8% of that "disappear" in failed charges (things like the card had expired). Of course, anybody can unpledge, but those numbers are reflected in the current total.
So if a friend of mine had a Kickstarter project and I wanted to get it some attention, I could use a credit card, donate $10k, and then cancel my credit card straight after? Kickstarter would show it as a pledge but will never get the actual money. It seems like very dangerous idea of course =)