Open borders have been a matter of discussion within the libertarian movement for a few years. Bryan Caplan, on this same blog, has consistently advanced a very strong case for open borders. See, for example, his "Let Anyone Take a Job Anywhere". If you can read only one thing on this issue, it really should be that post by Bryan.
(...) a little reflection should tell us that the key is to control not so much movement across borders as what people do within borders. It's really not simply about numbers, and certainly not about foreigners crossing borders.
Regulating immigration is not just about how people arrive, but about what they do once they have entered a country. It is about controlling how long people stay, where they travel, and what they do. Most of all, it means controlling whether or not and for whom they work (paid or unpaid), what they accept in financial remuneration, and what they must do to remain in employment, for as long as that is permitted.
Yet this is not possible without controlling citizens and existing residents, who must be regulated, monitored and policed to make sure that they comply with immigration laws.
If immigrants must show their passports at borders, everyone will have to, including returning citizens. If immigrants must present their credentials at internal checkpoints, then everyone, including citizens, will have to do so -- if only to prove that they are not immigrants.