The elegance of the Flat Tax idea is not the flatness of the tax, but the fact that it's a straight line tax.
There's no reason why this essential feature can't be combined with a progressive (read: more humane) system by adding a gentle slope.
For those who care about progressive vs regressive - the fairtax proposal actually proposes a pre-bate to families to allow for income disparities and the affect on lower income families.
One would think it would be possible to create a similar pre-bate system to assuage concerns for a flat tax.