Right. I think people are getting hung up on the GP's incorrect use of "duress". There are a bunch of reasons why a contractual clause might get thrown out: there's the public policy rationale, as you note, there's undue influence (though judging from Wikipedia, that might not work in the U.S., as both cited cases are Australian), and there's my personal favorite:
Unconscionability. It's a two-prong attack:
1. You have to prove that whatever your employer offered you in exchange for signing the contract (viz., a job, at a certain rate of pay, for certain hours) was so grossly inadequate compared to what you had to give up, that to enforce the contractual terms would be unfair.
2. You have to prove that your employer leveraged his greater bargaining power to get you to sign the contract.
Essentially, you have to prove that both the terms of the contract and the manner in which it was negotiated were both grossly unfair. #2 is a lot easier to prove than #1, given the state of the economy and the desperation of many job-seekers, but #2 alone isn't sufficient. In order to break a contract on unconscionability grounds, the terms have to be really, really onerous, on the order of "you agree to give up your firstborn for indentured servitude to this company" or "if you quit, you agree never to work within a fifty-mile radius of this company ever again".