I've argued several times that open immigration is an idea whose time has come (below, for example). Austin the Undergrad disagrees and raises a number of good points, but a) I think that the empirical literature still suggests a net gain and b) his criticisms aren't criticisms of immigration per se but of the welfare state (I discuss discrimination and forced association here). The problem is that for open immigration to work to its fullest potential, a lot of things would have to change at once.

Ilya Somin links to an interesting piece by Paul Romer and offers intriguing commentary (HT: Will Wilkinson). I share their concern that a massive humanitarian/military/colonial/whatever exercise in state-building is a recipe for disaster.