Should current trends continue -- from higher payroll taxes to the potential impact of the Alternative Minimum Tax on middle-class earners -- the US system could tip from progressive to flat in a matter of years, at least for the top half of earners. And then tip back again. For decades, the American tax burden has been shifting away from the rich. Now, two economists say, it could be at the brink of a historic tipping point, [looking at how taxes] affect Americans at each income level, and how tax burdens have changed over the past generation. Their short answer: The rich are paying less than ever. See the April 15 Critical Faculties column in the Boston Globe.