Withholding probably won't change, that's usually based on your W4 you submitted for this year.
In reality, every additional taxable dollar you earn is taxed at your marginal rate, whether it's at a higher rate than your other recent pay is mainly based on whether you were right at crossing over into the next tax bracket or not. [i.e. the $s earned in your last few months were all in the 12% bracket, but the bonus $s filled up the last of that bracket and those above got taxed at the 22% level]
Reasons you might actually pocket more $ from this bonus:
1) you were right at the cusp of the OASDI ceiling, and no longer have to pay the 6.2% part of the payroll tax on these additional dollars
2) your benefits withholding, like health insurance, etc., is generally not taken out of bonus, just regular pay
3) you had not maxed out 401k or other tax-deferred savings plans, and are able to direct the money into those and not have to pay any income tax