see his #7. I guess it’s definitive.
Tax rates will go up for tax years beginning AFTER 12/31/10, if the tax cuts are not extended. They will still be in effect for the 2010 tax year.
Keep in mind that since withholding was reduced last spring to offset the $400 tax credit for 2009, and there is no $400 tax credit for 2010, then your withholding will go back up to what it was before it was reduced last spring.
The other thing that may increase your withholding if you didn’t see an increase in gross pay would be if you had a DECREASE in pre-tax items (such as amounts withheld for 401k, section 125 cafeteria plans, a lower amount withheld for your portion of health insurance premiums (not likely), etc. These amounts are deducted from your reported taxable wages, so if pre-tax amounts went DOWN, your reportable taxable income would go UP, therefore your tax liability would go UP, therefore your withholding would go UP.
I would check with your payroll department, just to make sure they are using the right W-4 exemption amounts.