The U.S. Department of Labor has put together a new section on their web site with "Off-the-Clock" wage and hour information sheets: http://www.dol.gov/esa/whd/offtheclock/index.htm.  The information concerns how an employer determines what are "hours worked" which then determines the overtime and minimum wage obligations. 

For example, if an employee shows up to work and is told immediately that there is no work available that day and is sent home by the employer, the employee is not considered to have had any "hours worked."  However, if the employee is required to wait until after the shift begins to have the determination made, they are considered to have "hours worked" for the time they waited until they were told no work was available, and wages are owing. 

Specific information sheets are available for different occupations and industries including construction, restaurants, hotel and motel, manufacturing, and several other areas.