A couple factoids from the IRS website:

Foreign income. If you are a U.S. citizen or resident alien, you must report income from sources outside the United States (foreign income) on your tax return unless it is exempt by U.S. law.

Gambling winnings. You must include your gambling winnings in your income on Form1040, line 21. If you itemize your deductions on Schedule A (Form 1040), you can deduct gambling losses you had during the year, but only up to the amount of your winnings.

And, in case this applies to anyone (I'm sure the answer is NO)…

Illegal income. Illegal income, such as money from dealing illegal drugs, must be included in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity.

I'm sure they get a lot of compliance on that one…