So you can see that it phases out gradually. It works out that your marginal rate in the phaseout range is quite high, but at no time is it higher than 100%.

Let me try and find a graph for that.

ETA: Here's what I have. Let me explain a bit:

Effective Tax Rate: Add up all your money and divide by the amount of tax you pay. This is the number you look at and yell at the darned liberals for stealing your money.

Marginal Tax Rate: The % tax you would pay on the next dollar you earned. This is the important number in determining how worth it it is for you to earn.

Here's a chart of how tax brackets work:

But then there is also the EITC, which gives you a payment based on the amount you earn at lower incomes, so you have to subtract this EITC payment from your Marginal rate.

As you see, it phases out. So when you earn more money at that point, not only are you paying tax on that extra money, but it reduces your EITC payment, so you get hit twice! When you add it all up, it looks like this:

Note that the marginal rate starts at negative, since you get back money from the govt in the form of EITC. It goes up to 40% at the EITC phaseout, and then drops back to 10%, before rising again. But, for you to actually lose money by making more money, you'd need to have a Marginal Rate of over 100%, which never happens, because the tax lawyers who write this stuff don't want that to happen.

The bizarre part of this, is that you do have a Marginal Rate of 40%, and you are also losing other benefits like Food Stamps, so it is rarely worth it to take a small promotion and earn a few more bucks in that range. You should only advance if you can make a real jump.