from the IRS website

When and how will people get the $400 to $800 “Making Work Pay” tax credit? Taxpayers will not get a separate, special check mailed to them like last year’s economic stimulus payment.
For many taxpayers, the additional credit will automatically start showing up in their paychecks this spring. For people who receive a paycheck, the credit will typically be handled by their employers through automated withholding changes. For some other people, the credit can be claimed when they file their 2009 tax return next year.