You can count me into the people who support flat tax rates, if for no other reason to stop the pointless millions of hours of productive time wasted trying to rort the system. While people will always cheat, if all income is taxed at a flat rate, at the source, then the incentive to spend money on tax accountants falls.
The Negative Income Tax seems like an interesting flat-tax proposal that could even satisfy those in favor of progressive taxation for political reasons: http://en.wikipedia.org/wiki/Negative_income_tax
See also the Bradley X Tax, which is a value-added tax melded with an income tax. The income tax can be as progressive as you like, and the VAT is equivalent to a flat sales tax.
And what will you do with the half of the population whose total annual earnings don't meet their tax obligation? That's a lot of soylent green for the rest of us to eat.
In the marginal or total revenue? Ie tax = C, or tax = c * income?
How would we define fair here? Both of the above options seem somewhat fair: everyone contributes the same amount, or everyone contributes the same percentage of income. Or a third standard for fairness could be that the benefit you get from government programs is equal to what you pay in taxes.
I don't even know which of these I think is best (let alone a hope of ever changing anything). Perhaps the best way to cash out "fair" in this case would be to start with some assumptions about what taxes are supposed to do; eg solve coordination problems, support disadvantaged people, etc; and see what can be derived.
