It could even be helpful, at least according to the American Optometric Association. The AOA asserts that "3D viewing may actually help uncover subtle disorders that, left uncorrected, often result in learning difficulties."
That isn't to say that playing a Nintendo 3DS and seeing how a child responds is in any way a replacement for formal eye exams. But it could help spot warning signs, such as discomfort, dizziness, or lack of depth.
Furthermore, while the AOA doesn't advise parents to just stick a Nintendo 3DS in front of all kids, the organization does help Nintendo out by stating that "children younger than 6 can use the 3DS in 3D mode if their visual system is developing normally."
This is surely going to become a bigger topic once the Nintendo 3DS launches in Japan next month, followed by Europe and the United States in March. Do you plan on buying a Nintendo 3DS?