Apparently whether dog or mouse is the outgroup was under debate...pretty interesting question when you think about it. Some thought that mouse should be an outgroup to human and dog, but this person doesn't think the phylogeny (dog, (mouse, human)) needs to be reconsidered.
Dog as an Outgroup to Human and Mouse
PLoS Comput Biol 3(4): e74
Abstract: In a recent contribution to PLoS Computational Biology, Cannarozzi, Schneider, and Gonnet published evidence that rodents form an outgroup to human and dog , in disagreement with several recent studies suggesting that the dog is an outgroup to the primate–rodent clade [2,3]. The authors' arguments rest on a variety of analyses of human, mouse, and dog genes, using opossum to root the phylogeny. Here I argue that despite the large number of characters used in this study, their results may well be erroneous. I then provide new and, I believe, conclusive evidence in favour of the current consensus phylogeny, and I briefly review other recent studies that support this conclusion.