From ObamaTaxCut.com:

All numbers are from the non-partisan Tax Policy Center. Obviously, you’ll note that filling out your taxes is a lot more complicated than just listing your income, filing status, and number of children — these are estimates based upon the stated policies of the candidates and average deductions claimed by folks in similar economic conditions. Luckily, Obama promises to simplify the filing process by filling forms out for you. The number reported above is the estimated difference between income taxes paid under George Bush’s current policies and the the taxes under the Obama plan.