I get a call at least once a month from a panicked client who just heard a rumor that they have to give breaks to employees every day. Unfortunately, there is no snopes.com for employment myths, so this almost total fallacy gets perpetuated over and over. In the vast majority of circumstances the answer is: NO.
Of course, no legal answer would be complete without exceptions. As of May 2010, nursing mothers are now guaranteed a break under Obamacare. Employers have to provide a private space (other than a bathroom) and give mothers time to express milk. On a separate note, the City of Austin, Texas recently passed a city ordinance requiring a 10 minute break every 3.5 hours for construction workers after three men died in 2009.