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:
Elka does not "smile" in the "mistake it for a snarl" kind of way. She will do a similar thing, where her little lips are puffed over her canine teeth. My fiancé insists that her lips simply got stuck when she picked up or dropped a toy. I've seen her do it with no toy involved. But, that's the way those things go sometimes, amiright?
(you can kind of see the puffed lips here a little bit, but this is definitely more a "traditional" dog smile)
I finally got a picture of her doing it, though it's one of the worst pictures ever, of course. The laser eyes don't really help. This was one of the "no toys involved" times it occurred.
I've tried to reinforce it, call attention to it, so that she knew I liked the behavior. It's been easier teaching her to Sneeze on cue, in fact. It's just a spontaneous behavior, one I just need to enjoy when I see it. Because (despite the weird picture above), it is just so darn cute!
As a bonus, Jesse the smiling Doberman from Youtube (who is not my dog. Jesse the smiling Doberman is simply a wonderful example of the smiling I'm discussing).