> Today even the poorest Americans likely end up short more
> than 2.4 percent (effective annual rate)
That really depends, due to the complexity of the tax code. In particular, depends on whether you're married, whether you have children, etc.
You can see some historical data for 4-person families at the median, half median, and twice median income levels at <http://www.taxpolicycenter.org/taxfacts/Content/PDF/family_i.... Note the negative numbers in the "average tax rate" column in recent years for the half-median level (due to the making work pay tax credit, in addition to the preexisting earned income tax credit and child tax credit). But yes, before that the average rate was higher than 2.4%. And this is federal only, not counting state and local taxes. (And the part that really makes me mad about the numbers in this chart is the marginal rate on the half-median income level, by the way.)
Of course if you're not married with children, your tax burden is definitely higher. For a single California resident in 2007 you can see a simple marginal tax rate graph at <http://www.dbaron.org/views/taxes-2007.html>; the average rate would just be the integral of the graph, which gets to be higher than 2.5% pretty early on.