The device is called the "NEBA system" (for Neuropsychiatric EEG-Based Assessment Aid). It measures the ratio of theta to beta waves (higher quotients are found in those with ADHD). An FDA official said the device "along with other clinical information may help health care providers more accurately determine if ADHD is the cause of a behavioral problem."
William E. Pelham, the director of the Center for Children and Families at Florida International University, which conducts research on the disorder and treats children who have ADHD is skeptical and dismissive. He told The New York Times that traditional diagnostic methods are relatively accurate. [My emphasis] "We're not going to run out and buy one of these machines to do diagnoses, because it is totally unnecessary," Pelham told the Times.
However, most parents don't have access to diagnostic centers and diagnoses are made by pediatricians and other family care practitioners. If the devices are not too expensive, perhaps they would be helpful to doctors who are not ADHD specialists. Ruling out ADHD is important, particularly if medication is going to be prescribed.
FDA news release (Free)
New York Times story (Free)