The distinction is who the right to the idea is enforceable against. A patent represents a government enforced monopoly that bars anyone from using the specific intellectual property, whereas a contractual right to an idea can only be enforced against the other party to the agreement.
In California, the seminal case is the Supreme Court's decision in Desny v. Wilder, which held that a screenwriter could enforce a studio's promise not to use one of his ideas without his consent. The Court's reasoning was that while a party is free to come up with its own ideas, the disclosure of someone else's idea confers a benefit. Federal courts have addressed this issue a lot in a line of cases holding that Desny claims are not preempted by the Copyright Act.
I haven't been involved in any cases where the alleged theft of an idea involved an employment contract, but I see no reason why the same principles would not apply. Logically, an employer should be able to disclose propriety ideas to its employees without fear that they will be stolen, regardless of whether the ideas are sufficiently novel to qualify for an exclusive federal monopoly.
The classic case in this area is Warner-Lambert v. Reynolds 178 F. Supp. 655 (1959). (It's about a contract covering the formula for Listerine)
http://law.justia.com/cases/federal/district-courts/FSupp/17...
(ps. Don't answer. This is the sort of thing to be discussed with an attorney behind a closed door.)