In 2006, a developer raised the question in the Amazon Discussion Forum whether there is a risk that the bandwidth cost grow beyond a level he was not willing to pay for. AWS answered that such a feature is in the work. The plan was to enable users to cap how much they are charged each month.
Three and a half year later, this is still a plan. The latest message from AWS was : later this year (2009) or early next year (2010).