How progressive is 'progressive taxation'?

More good stuff about the flat tax from Madsen Pirie at the ASIblog. The problem which he doesn't deal with, from a political point of view, is the perceived impact on low-income people just above the payment threshold who currently are eligible for a range of help in meeting their obligations (perhaps the elderly or retired, large working families). I suspect this can be shifted, but it is best to be honest about it from the start.