http://www.dol.gov/esa/minwage/q-a.htm

An employer of a tipped employee is only required to pay $2.13 an hour in
direct wages if that amount plus the tips received equals at least the
federal minimum wage [of $5.15] , the employee retains all tips and the
employee customarily and regularly receives more than $30 a month in tips.
If an employee's tips combined with the employer's direct wages of at least
$2.13 an hour do not equal the federal minimum hourly wage, the employer
must make up the difference.