(paraphrased for those of you in Rio Linda)
"Should a Fisher-style pure consumption tax be implemented, government could easily manipulate the retail prices of non-prebatible consumer goods for their own tax gain by import tariffs, de-zoning or banning increased production, filing lawsuits, restricting access to raw materials (such as logging and mining), restricting transport, or in the simplest form, raising the cost of transport and motor fuels. Political acts decreasing supply by artificial means, causing unconscionable increases in retail prices of said [necessary] goods, and the subsequent onerous levels of taxation.."
see http://www.mises.org/story/1768 for the entire essay.
We'll call that the "Something else bad might happen in the world so let's keep the IRS and the income tax argument."