The American Hosptial Association (AHA) released the results of its Annual Survey of Hospitals in October. In its Uncompensated Hospital Care Cost Fact Sheet, AHA said that the data from the Annual Survey showed that hospitals had uncompensated care costs amounting to $31.2 billion (5.7% of total expenses) in 2006. However, uncompensated care excludes other unfunded costs of care, such as underpayments from Medicare and Medicaid. In its Underpayment By Medicare and Medicaid Fact Sheet, AHA reported that underpayments to hospitals (defined as the difference between the costs incurred and the reimbursement received for delivering care to patients) reached $29.9 billion in 2006 (up from $4 billion in 2000).
Other reports and studies by the AHA can be found here.