The answer depends on where the employer wants to put the cameras and why. The employer must have a reasonable, legitimate business reason for monitoring employees in this manner (for example, to discourage theft from a cash register or to enhance the security of customers and employees). And some states have made certain areas of the workplace (for example, the bathroom and changing areas) off-limits to this type of monitoring.
Even with a legitimate reason, your employer must inform you and your coworkers that the cameras are there. Also, some states prohibit the use of certain types of surveillance devices, such as one-way mirrors.
For more information on workplace surveillance, see Nolo's article Cameras and Video Surveillance in the Workplace.