Thrombus is right... Luckily most non-compete clauses last only a year or so, and are restricted to a geographical location specified in the contract. You don't HAVE to have it in your employment contract, and it's something you can negotiate up-front. But if you did not you are bound by those limitations, and breaking the clause can cost you BIG.
I have not had to deal with this clause personally, but if it was presented I would attempt to strike it from the contract, or at least negotiate it down in geographic size such that if I did plan to start up my own business I could still operate in the same city (although maybe across town). I bet most employers would rather negotiate that clause than salary.