Alibaba filed papers with the SEC for what could become the largest U.S. IPO ever. The only number the Chinese online marketplace included in the filing was $1 billion. It is being used as a placeholder to calculate fees. Of course, many expect the amount of money Alibaba eventually raises will be closer to $20 billion. The SEC filing did not include numbers on how many shares it will sell, what valuation it is seeking or which exchange it will list on. Most of the big banks will participate.