With the government appearing to reach agreement on taxes for 2013 there should soon be an updated set of tax guides for use by employers in the new year. Publication 15 is the usual guide that companies use for their payroll withholding lookups and verification of employment rules throughout the year. Because of the uncertainty around the 2013 tax status many of these guides have not been updated (yet) for the coming year.
Check back at the link below as the updated tables should arrive soon in January 2013