I believe you may be misunderstanding the situation. Obama is proposing to increase the top marginal tax bracket to 39% above $250,000. This means that all income above $250,000 will be taxed at 39%. The tax on income below $250,000 remains exactly the same.
Thus, if you make $250,000, your tax will be some value (let's call it X).
If you make $250,001, your tax will be X + $1 x 39% = X + $0.39
All of your income below $250,000 will be taxed at each of the lower tax brackets. So if you decide to make $260,000, your additional tax would be $10,000 x 39% = $3900 instead of $10,000 x 34% = $3400 under current law, a difference of $500. The difference is small if you make $250 - $500K. Once you get over $500K, the difference gets larger.
... unless I'm misunderstanding the situation myself, which would not be uncommon