I think the marriage penalty has been reduced, but may still be there. Boxers who deal with Teh IRS on a more frequent basis probably know better.

One place where it gets screwy is when you've got two similar incomes. My wife and I were both making about the same amount for a couple of years pre-kids, and the payroll withholding tables didn't account for that. I was basically withholding at the single rate. I think we ended up owing Uncle about $2-3K the first year. (edit: looks like sdm1130 was in the same boat, maybe)

If I were you, I'd keep your filing status with your employer as single (for W-4 tax purposes) during the tax year you get married. You'll probably get a hefty refund, and then you can adjust your filing status/withholdings to get close to $0 owed/refund.