This is a condition that is commonly confused for a hernia by patients. It is not a true hernia but it can cause a bulge in the abdomen that can be quit alarming to a patient.
Patients will describe a bulge down the center of their abdomen which is only apparent when they are laying flat and start to sit up. At that point the abdominal wall muscles contract. There is a pair of very strong muscles going down the middle of the abdominal wall like a pair of pillars. If there is a large space between these two muscles the patient will notice a bulge between the muscles when they sit up.
This is not a true hernia and has none of the dangers of a true hernia. Usually it is not treated unless the patient wants treatment for cosmetic reasons. As one patient posted on our YouTube page, "Appropriate management consists of reassurance of the patient regarding the innocuous nature of the condition."
Update 1/12/2014 - Having recently seen a few patients with huge diastasis recti who have convinced me that they had abdominal discomfort as a result of their diastasis I can no longer say that repair of diastasis should only be done for cosmetic reasons.