I'm assuming what people are specifically referring to is pot growers in general and legal sellers in Colorado specifically.

In other words, a full time, legal, state licensed pot seller in Denver will be filling out taxes next year for the income made selling pot legally in 2014. State taxes are no big deal. But the federal government still considers it a crime. When the Denver, Colorado pot salesman puts down on his federal taxes the income he made by performing activity the federal government claims is illegal... then what? Is it conveniently ignored by all parties involved?

As far as the other comment... People who make a significant illegal income growing and selling pot in states where it is NOT legal to do so, either further risk the chance of going to jail (just like the example of Al Capone you gave) by not filing taxes, or else they file taxes and lie about their occupation, thereby causing the feds to collect tax on illegal activity all the time.