It actually appears on some embassy websites for getting visas to visit the US -

American citizens should bring two identical photos when they apply for a U.S. passport. The following minimum requirements must be met or your application will not be processed. Please see the paragraphs below for more details.

1. The photographs must not be older than six months
2. The photographs must measure 2x2" (5x5 cm)
3. The photographs of the applicant's head must be between 1" to 1 3/8" (2.5 to 3.5 cm) when measured vertically
4. The photographs must have a white background
5. Must face the camera directly, with both ears visible

But then they say it's preferable

It is preferable that ears be exposed.

Though in their seven page official guide to passport photographs, it doesn't mention ears, and uses at least one example of somebody whose hair covers their ears.