Mt Gox is the dominant Exchange that handles BTC to USD conversions. Given the recent spike in price, and that Mt Gox is the primary exchange that publishes this price, it's conceivable that a second set of books could be used to manipulate the public -vs- non-published value of the coins.
What are the methods in which an Exchange can steal from its customers? (dual set of books, etc)
How can consumers and businesses protect themselves from exchange based fraud?