SixOfNine is full of shit, and can't back anything he says up. Adam Smith did not favor progressive taxation. Period. He favored proportional taxation. A flat tax is proportional (everybody pays the same proportion of that which is taxed), a progressive tax is not. Case closed.
Read it:
http://www.marxists.org/reference/archive/smith-adam/works/wealth-of-nations/book05/ch02b.htm
The following chapter is linked downpage from my link. He covers rents, income, profit from investments, consumption, and land taxes.
Smith rejects income taxes on capital, wages, and professional income.
http://www.marxists.org/reference/archive/smith-adam/works/wealth-of-nations/book05/ch02b-3.htm
Wages:
"If direct taxes upon the wages of labour have not always occasioned a proportionable rise in those wages, it is because they have generally occasioned a considerable fall in the demand for labour. The declension of industry, the decrease of employment for the poor, the diminution of the annual produce of the land and labour of the country, have generally been the effects of such taxes. In consequence of them, however, the price of labour must always be higher than it otherwise would have been in the actual state of the demand: and this enhancement of price, together with the profit of those who advance it, must always be finally paid by the landlords and consumers.
A tax upon the wages of country labour does not raise the price of the rude produce of land in proportion to the tax, for the same reason that a tax upon the farmer's profit does not raise that price in that proportion.
Absurd and destructive as such taxes are, however, they take place in many countries."
He only favors taxing the income of officials in goverment.
He favors a tax system that is in proportion to income, but not a tax on income itself. He also argues against a hard to understand tax system, like our own, where you have to hire professionals to figure out what you owe and even then good luck.