From Tax Prof blog:
TIGTA: Obama Tax Credit to Leave 15.4m Workers With Additional Tax Bill
The Treasury Inspector General for Tax Administration has released Millions of Taxpayers May Be Negatively Affected by the Reduced Withholding Associated With the Making Work Pay Credit (2010-41-002):
The Making Work Pay Credit, a provision of the American Recovery and Reinvestment Act of 2009, will apply to most taxpayers with earned income. The credit will be in effect for Tax Years 2009 and 2010. The Making Work Pay Credit was implemented using new income tax withholding tables issued by the IRS. Application of the tables could negatively affect a significant number of taxpayers. The overall objective of this review was to assess IRS efforts to implement the Making Work Pay Credit and to evaluate its impact on taxpayers. ...
Based on an analysis of Tax Year 2007 tax return data, TIGTA estimates that more than 15.4 million taxpayers could unexpectedly owe taxes for Tax Year 2009 as a result of the Making Work Pay Credit.
Associated Press, Millions Will Have to Repay Part of Obama Tax Credit
Bloomberg, Obama Tax Credit Leaves 15 Million Owing IRS: Report
L.A. Times, Millions May Have to Repay Part of Obama Tax Credit
Wall Street Journal, Millions to Owe IRS Due to Stimulus Credit -- Treasury