From the Wall Street Journal (hat tip to Sean for bringing this story to my attention): The Holdup at Online Banks (link here)
The banks say they want to avoid fraud, such as transfers from bad accounts, or when someone else gets hold of your online sign-on name and password and tries to move your money somewhere else. According to numbers compiled by the American Bankers Association, about $969 million was lost to fraud in 2006, the most recent year available, out of about $41.7 trillion in checking-related transactions, a number kept very low in part because of aggressive risk-management practices. But even when attempted fraud is factored in, more than 99.9% of checking transactions are good.
At the end of the story, the Journal tells you what to do if you want to move money between different institutions.