According to Section 9 of Publication 15 (Circular E) Employer's Tax Guide (PDF) as of 01/26/06:
The amount of any federal income tax withholding must be based on marital status and withholding allowances. Your employees may not base their withholding amounts on a fixed dollar amount or percentage.

However, an employee may specify a dollar amount to be withheld in addition to the amount of withholding based on filing status and withholding allowances claimed on Form W-4.

For additional information, refer to the Internal Revenue Services's web site