I see one problem with the "flat tax". It's that the money is WORTH much more to a person who has much less of it.
To a person who makes $10,000 a year, every cent is valuable, as they're barely getting by. But to a person making $100,000 a year, alot of that is disposable income.
In other words, the person who makes $100,000 a year can afford to lose $10,000. The person making $10,000 can not afford to lose it.
I guess, then, I'd be ok with a flat tax after a certain amount... perhaps the basic amount to survive decently. (I have no idea what that would be, offhand.)