From the US Department of State web site
4. Photographs.
You must present two identical photographs of yourself that are sufficiently recent (normally taken within the past 6 months) to be a good likeness. Passport Services encourages photographs where the applicant is relaxed and smiling.
The photographs must not exceed 2x2 inches in size. The image size measured from the bottom of your chin to the top of your head (including hair) must be not less than 1 inch nor more than 1-3/8 inches with your head taking up most of the photograph. Passport photographs may be either black and white or color.
Photographs must be clear, front view, full-face, and printed on thin, white paper with a plain, white or off-white background. Photographs should be portrait-type prints taken in normal street attire without a hat and must include no more than the head and shoulders or upper torso. Dark glasses are not acceptable except when worn for medical reasons. Head coverings are only acceptable, if they are worn for religious reasons.
Applicants may use photographs in military uniform only if they are on active duty in the U.S. Armed Forces and are proceeding abroad in the discharge of their duties.
From the Canadian Globe and Mail newspaper
Close your mouth. Look into the camera. Don't smile. Don't frown.
Canadians applying for passports must now submit photos showing "neutral expressions". They cannot be smiling, frowning, glaring or grimacing, the Canadian passport office said.
The changes follow recommendations from the International Civil Aviation Organisation, the UN agency responsible for aviation issues, aimed at making it easier for security personnel to recognise passport holders around the world, a spokeswoman told the Globe and Mail newspaper.
The measure took effect on August 15, but the office will accept photos that do not meet the criteria until November 3. After that, all passports issued will be smile and frown-free.