Cloning, simply defined, is creating a new organism that shares the same genetic code as another.  Each step taken toward making cloning quicker, better, and cheaper brings these ethical questions further from the realm of the hypothetical and closer to the realm of fact.

Cloning Endangered/Extinct Species
        All endangered species have one thing in common -- there's not many animals left.  A lack of animals leads to inbreeding, and a lack of genetic diversity.  Researchers are always careful when trying to reestablish endangered species to maintain genetic diversity by creating different families and trying to inbreed as little as possible.  If the scientists just let the animals breed without control, there will be almost no genetic diversity, which will lead to recessive traits being expressed.  Usually, this results in diseases and deformities.
        While cloning offers a short-term answer to the problem of "not enough animals", in the long term, all the offspring from the clones are genetically brothers and sisters.  This will likely destroy genetic diversity and lead to inbred, weak, disease ridden animals who could not survive in the wild.  Even if the animals were healthy enough to be released into the wild, in their weakened state, they would be easy prey, leading to more predators.  In the end, they would become extinct and would have upset nature's balance by introducing more predators.  The morality of cloning endangered species hinges on how many animals are left, and what their collective genetic condition is.
        The same trouble would be compounded if science tried to bring back an endangered species like the dodo bird.  It would be so expensive to find enough intact dodo cells from different birds, that to make a flock of unique birds would be an impracticality.  The best science can offer is the same cloned dodo at every paying zoo.  Whether or not you think this ethical hinges on the question of whether it is ethical to create an animal for the sole purpose of captivity.

Cloning Humans
        It would clearly be unethical to clone humans and distribute them about for the purpose of a gigantic twin study, to determine once and for all what is nature and what is nurture.  It would be useless for an millionare to clone himself in hopes of everlasting life; the clone would posess none of his memories or experiences.  Bill Gates' clone might turn into an average person, a delinquent, a ditch digger, or the secretary of the Department of Justice, depending on the clone's life experience.
 

When does life begin?

        There is really only one apparent and plausible reason for the widespread cloning of humans, and that is for parents who can't have children naturally.  How you feel about cloning humans really depends on how you feel about the value of human life, and when it starts.  Dolly took 277 tries; assuming that cloning a human would take 277 tries, what of the 276 human embryos that died?  If they are just embryos, then it's probably okay.  If they were humans with minds, souls, and destinies, then it was wrong to try to clone a child in the first place.  Cloning, it seems, offers the same ethical dilemmas as does abortion and current methods of in vitro fertilization.
        But there is a second problem on top of the consideration of the other embryos -- the life of the clone.  Assuming that the technique is commonplace (the lives of the first fifty or so will probably be filled with doctor visits, disgruntled family members, and possibly taunting classmates), a cloned child would have a set of problems in addition to the ones that normal children face.  What would be the relationship between the clone and the "DNA donor"?  Sibling?  Child?  Most children have problems with their parents; a clone might reflect those feelings back on himself, because he is identical to them.  There is a whole list of questions that prospective parents would have to consider before trying such an endeavor; is it really worth it?

Core Issue
        The core issue behind cloning seems to have to do with the relationship between genetic uniqueness and personal individuality.  Animals may or may not have their own personalities and sense of individuality, and if they exist, they may differ from species to species, so there may never be a resolution to the question of cloning animals.
        But humans are a different matter entirely.  Humans are unique not because of their body build or genetic makeup or life experience, but because of the unique contribution they each have to make to the world.  The morality of human cloning lies not in the cloning process itself, but in societal reaction to him.  If society can keep itself from branding the clone as a duplicate person and limiting him to his predecessor's abilities; if instead the clone is accepted just as a normal human being who is unique because of his unique contribution he has to make to the world, then cloning of humans may be an option.