] At the moment, homeowners can deduct interest they pay on
] their mortgages from their income taxes. Without an
] income tax, this subsidy, long seen as part of what made
] the American dream come true, would disappear. Bringing
] it back, perhaps through direct grants, would require
] raising the national sales tax rate to 21 percent.

a great analysis on why flat taxes, whether income or sales, would not work very well.

Economic View: What if a Sales Tax Were the Only Tax?