There are some variables from state to state but here is the federal take on it from the US dept of labor
http://www.dol.gov/whd/regs/compliance/whdfs22.pdf
Waiting Time: Whether waiting time is hours worked under the Act depends upon the particular circumstances. Generally, the facts may show that the employee was engaged to wait (which is work time) or the facts may show that the employee was waiting to be engaged (which is not work time). For example, a secretary who reads a book while waiting for dictation or a fireman who plays checkers while waiting for an alarm is working during such periods of inactivity. These employees have been "engaged to wait."
On-Call Time: An employee who is required to remain on call on the employer's premises is working while "on call." An employee who is required to remain on call at home, or who is allowed to leave a message where he/she can be reached, is not working (in most cases) while on call. Additional constraints on the employee's freedom could require this time to be compensated.