Most proposals to replace the income tax with a sales tax include safety valves for low-income people. Current sales tax laws have incredibly complex exceptions for things like food.

Simpler proposals involve a nice, fat tax rebate or credit for lower income people, like the earned income tax credit. So they pay the simple "applies to everything" sales taxes, but get a big chunk of money back to compensate. It's a little more roundabout, but the end result should be paying little or no taxes.

The appeal of a sales tax is that it's simpler, short-circuits the "underground economy," hits the people who spend the most, and does not require one to list their entire life on paper and send it in to the government every year. (Ever wonder why so many people who are incensed lately about the government invading our privacy seem to have no problems at all with this?)

The reason it will never happen is that current tax law is a major source of political power. It's a way to reward friends, and punish enemies, and politicians will not abandon a tool like that lightly.