In Russia this already happened with their "state farms":
> a system of internal passports prevented movement of employees and members from rural areas to urban areas. In effect farmers became tied to their sovkhoz or kolkhoz in what is described by some as a system of "neo-serfdom".
There's 20 million people living in Beijing though, which allready makes it a transportation and housing nightmare, with prices for rent skyrocketing. If you've been to beijing around Spring Festival, where most people head back to their families, you'll notice that it is almost empty. Like positively razed of people. In a way, yes, controlling such migration is a horrible thing to do, but I couldn't imagine Beijing with another 20 million migrant workers trying to crowd their way through the city, along with the major Spring Festival rush that'd happen.
Right, I was going to mention this but you beat me to it.
My impression was that in reality, you rarely get checked unless you're in a region with stability concerns (western Sichuan, Tibet, Xinjiang), near a protest or terror incident, doing something official, etc. But it's a scary system to have hanging over you. Generally people can normalize with the support of an employer.
In some cities, like Shanghai a few years back, I've heard that the police do make more use of it to give immigrating rural people problems.
> a system of internal passports prevented movement of employees and members from rural areas to urban areas. In effect farmers became tied to their sovkhoz or kolkhoz in what is described by some as a system of "neo-serfdom".
http://en.wikipedia.org/wiki/Sovkhoz