Nature Methods just published a paper that uses careful descriptions of grimaces, e.g. "the eyes close and the area around them tightens", which were originally developed to estimate pain in infants, to calibrate pain in mice. (We want to know how much what we're doing to them hurts, often because we're testing painkillers.) This is (explicitly) a followup to one of's books, The Expression of the Emotions in Man and Animals. The modern version of that is the "Mouse Grimace Scale (MGS)": sparing of text, chary of assertion.
Langford et al. say "there has been no study of facial expressions of pain in any nonhuman species." In 130 years! (but there is a 2003 conference volume on the expression of animal emotions.) Of course, we spent a lot of the intervening time refusing to believe even that infants could feel pain, and had to back into the admission that animals can. I suppose we went through denying that infants and animals could feel pain while accepting the stricture that, if they could, it would be wrong of us to inflict it; the latter not widely accepted in Darwin's day; history bends towards justice in a very gradual curve.
I can't think of an argument for believing that animals didn't feel pain that doesn't rule out the belief that other human beings feel pain. Other humans may say so, but then, we lie.
Langford, D. J. et al., "Coding of facial expressions of pain in the laboratory mouse", Nature Methods, doi:10.1038/nmeth.1455 (2010).
Project Gutenberg text 1227, Charles Darwin, The Expression of the Emotions in Man and Animals, 1857. (Good line drawings of dogs -- it's a very English book, as I recall.)So wrote clew in Science.