You don't have to agree to sign such agreements. Once a company has invested in hiring you they aren't likely to let you go when you're at the proverbial starting line. Its more a case of who is going to blink first.

I just know that I've refused to sign more than once. Do you think, especially in Silicon Valley, that the employer not only wants to lose the employee but be bad mouthed about restrictive agreements online where it's likely they will lose even more battles in the recruiting wars?