Well, in general, if you have to "give" a creature rights, as opposed to the creature being able to "claim" rights, then it's questionable as to whether that being has any inherent rights.
But I have the same problems Greg has; there are many humans who are not capable of understanding the concept of rights, and yet it would be considered very immoral to treat them like animals. Mentally retarded, small children, etc. Of course, this could be the starting point for an abortion debate, but I don't want to go there...
Although I can't nail down the firm limits to my philosophy and therefore have no firm basis for what I believe on this subject, I can say that I don't believe in rights for animals. At the same time, I also think it's immoral to be cruel to them.
For instance, I'm okay with slaughtering cattle for food and leather, and harvesting deer for sport and wildlife managment, so long as a an effort is made to avoid torturing the animals.
If you tried to nail me down on why I think this way, I would say that even though an animal has no rights, being cruel to it has a negative effect on humans due to the empathy we feel for creatures which are clearly aware and capable of feeling happiness and pain. It's demoralizing to us, basically. (At the risk of anthropmorphizing, I suppose.) Don't know if this makes sense, but there you have it.
After all, I have a pet, and I would react violently were I to see someone abusing my pet, and my reaction would be based on something substantially more than the fact the animal happens to be my property. I still don't think the animal has rights, per se.