@Robert Gentel,
I'll tell you what the single biggest influence I've seen lately on people's attitudes regarding breeding of pets is, and I find it both reassuring and disheartening: Oprah Winfrey talking about it. She spoke out on puppy mills on a show (or something; I can't say I really follow the woman's actions), and there was a visible ripple among her audience of "Oh, I'm going make sure I don't get one of those animals." I heard people talk about it, a friend of mine who works at one of the more economonical clinics in town saw it -- when that woman talks, people listen.
And that's what it comes down to, I think -- getting people to change their attitudes without coercion. Dog licenses in our county are more expensive for unneutered animals than they are for neutered animals, but the difference is a small percentage of what it would cost to have the surgery done at a private clinic and, anyway, the people most likely to register their animals are the people who are least likely to be allowing their animals to be bred, whether or not they have them fixed. You could, I suppose, make the difference much greater, but that's just going to result in people not licensing their animals at all. You could, I suppose, impose stiff fines for not registering animals, but there's not really personnel available to step up enforcement and whatever local leaders put it in place would be sure to lose their seats at the next election...
What we do -- the low-income spay/neuter programs we run -- probably could have a sizeable impact if they were operated on a much larger scale, but we already encounter some understandable friction from small clinics who see us as cutting into their bottom line. It's actually the impression of a lot of people in this line of work (and I think has been supported in a handful of small studies, but don't quote me on it) that people who have their pet spayed or neutered, even reluctantly, are more likely to pay for veterinary care for their animal in the future, but that's bound to sound specious to the clinic owner who can barely stay afloat and who makes needed money from spay and neuter surgeries. And the last several months have been
very hard on veterinary clinics. And the grant money that allows us to subsidize the surgery we are doing can only be spread so thin before we start to lose money ourselves...
At any rate, I really don't think there is a top-down, government-mandated solution to the problem of pet overpopulation. I think it has to start with changes in attitudes at every level of society that it is shameful to have your intact pet wandering the neighborhood, or to be breeding in your backyard because puppies or cute or pit bulls are badass or whatever, or to buy an animal at a store (which, thanks in part to Ms. Winfrey, is more and more unfashionable already), or to drive across two states to get a Labrador retriever from a particular breeder when there are bunch of them sitting in runs at the shelter across town.