However, you can't take trade secrets or sales lists with you, that kind of thing. But you cannot be prevented from (say) working at software company A, then walking across the street to work at software company B doing the same job.
The trade secrets thing is where it gets nasty. I've seen people in court because the old employer accused them of using trade secrets in the new job. So if your old employer has a tendency to be nasty, you need to be squeaky clean about your departure, and even then you might get sued.
