The income tax is unconstitutional? Bull.
Amendment XVI
The Congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several States, and without regard to any census or enumeration.
Anyone who tells you otherwise is lying and probably trying to sell a book.
The argument I've heard against the IRS is that this Amendment was not properly ratified by some of the states.
I am sure that you would agree that the government over- or unequally taxes its citizens. Unconstitutionality could be based in equal protection. By subjecting one group of citizens (say, high-incomer earners) to a significantly greater tax burden could be considered discrimination. If we were to tax (fill in an ethnic group, say Hispanics) more than Whites, it would be obviously unconsitutional.
As far as equal protection is concerned, the government is clearly discriminating against high-income earners. While the amendment itself might be constitutional, the the current structure and execution of the income tax is unconstitutional.