From Marginal Revolution:
And no I do not favor open borders even though I do favor a big increase in immigration into the United States, both high- and low-skilled. The simplest argument against open borders is the political one. Try to apply the idea to Cyprus, Taiwan, Israel, Switzerland, and Iceland and see how far you get. Big countries will manage the flow better than the small ones but suddenly the burden of proof is shifted to a new question: can we find any countries big enough (or undesirable enough) where truly open immigration might actually work?
In my view the open borders advocates are doing the pro-immigration cause a disservice. The notion of fully open borders scares people, it should scare people, and it rubs against their risk-averse tendencies the wrong way. I am glad the United States had open borders when it did, but today there is too much global mobility and the institutions and infrastructure and social welfare policies of the United States are, unlike in 1910, already too geared toward higher per capita incomes than what truly free immigration would bring.
Plunking 500 million or a billion poor individuals in the United States most likely would destroy the goose laying the golden eggs. (The clever will note that this problem is smaller if all wealthy countries move to free immigration at the same time, but of course that is unlikely.)