Why the state withholding? Would the NRST knock out state sales taxes and so state withholding would be the only means of financing state government? Or do you mean that the NRST is just to replace any and all of the current Fed tax system?
I'm asking this very seriously, because now I'm a bit confused.
The NRST is a federal tax, replacing only federal taxes. If a state still wishes to continue with a state income tax (which would be a bit more difficult without the federal IRS to piggy-back off of), they are free to do so.
Of course, without the infrastructure for income reporting the IRS provides, the states may choose to either replace or dramatically simplify income taxes.
The NRST would be the only federal tax so your pay check would have no federal withholdings. State tax withholdings and state tax laws would remain unchanged.