They are not in violation of the constitution, as of the passage of the sixteenth amendment:

Sixteenth Amendment to the United States Constitution - Wikipedia, the free encyclopedia

Sorry.

Income taxes started at a measly 2% or so, and have ballooned to around 30% on average. It's hard to stick a government with breaking rules that they wrote themselves though. The trouble is, our federal government is so huge and bloated, and the guys in charge want to keep it that way. That's why they're not letting Ron Paul into debates, for instance.