This is an unsettling thing to the unprepared owner. If you're not versed in the package of canine body language, such smiles might look like snarls. It is a lifting of the lip and a baring of the teeth. But there is no piloerection, no stiffness of posture; when I've seen it in pictures and video, it's been accompanied by body wiggles and airplane ears.
Of course, there are generally accepted "dog smiles" as well:
