I'm not sure about this. I saw an interesting episode of the BBC series Horizon a while back in which Danny Wallace tried to answer this question. A couple of the philosophers he spoke to were very dismissive and didn't even bother to address the question properly, but I thought the part where he met a chimp and she asked him to play chasey with him was quite lovely.
When we talked about this subject at uni the conclusion I came to was that animals should be treated more or less like children, ie. looked after but not expected to take full responsibilty for their own actions because they just don't have the same intelligence that we do. Looked at in a historical context though that sounds a bit creepy since there was a time when women and people of particular races were regarded as things or as children rather than fully fledged thinking adults.
I would think a big stumbling block would be that it's hard to measure the intelligence of a creature who is very different from us since they don't see the world the same way we do (literally even, some have different senses from us).
So...yeah, I really don't have an answer, but I'll be interested to see what other people come up with.