Miles,
I am guessing you are already aware of all the arguments about suffering, the presence of central nervous systems, some indication of avoidance of pain, etc. We have also hashed over the various arguments (especially those of Singer(*)) regarding the legitimacy and basis of extending ethical considerations across species while still differentiating between them. So let me ask you a question that may help us get a grip on this: I assume you feel less queasy about kicking a leaf (or even better: a rock) rather than a dog. Do you think that is just a programmed response or aesthetic issue? Or do you think you are acting based on some considered position (apart from pragmatism/logistics)?
--ravi
(*) At the cost of misrepresenting him, I can offer a very simplistic summary of Singer's argument: we derive human ethical rules that govern our action on the basis of some universalisation (such as the categorical imperative). For instance, why do we hold that all else equal, kicking another human is wrong? Singer argues that when we flesh out these notions we will find that there is no logical basis to reject extending such considerations down the hierarchy of animals that share similar conditions and therefore the universalisation.