http://en.wikipedia.org/wiki/Flat_tax
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.
http://en.wikipedia.org/wiki/X_tax http://www.princeton.edu/~ceps/workingpapers/93bradford.pdf
A flat tax is more fair than a progressive one. But it would be more fair with a head-tax aka. poll-tax. Everyone pays the same amount.
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.