One other thing I forgot to mention is that the ultra-wealthy typically earn most of their money from investments, not income. This is why Mitt Romney pays a net lower tax rate than you or I and another reason why even progressive income taxes are fairly regressive.

If the ratio of taxes to income isn't rising with income or at least keeping constant (which is what I think you're referring to as "linear"), your marginal tax rate is declining as income rises. I think you maybe mean it's staying constant with consumption, but that's kind of the problem - people with higher incomes tend to consume a smaller portion of their overall income, so taxes based on consumption are usually regressive.

That's pretty much the definition of regressive when it comes to taxes - people making more are being taxed less, proportionally.

http://en.wikipedia.org/wiki/Regressive_tax