In the '2006 Taxable and Nontaxable Income' document (publication 525), the IRS requires you to include valuations for the following:
Stolen property. If you steal property, you
must report its fair market value in your income
in the year you steal it unless in the same year,
you return it to its rightful owner.
Bribes. If you receive a bribe, include it in your
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 guess the Government have all angles covered...