[kls]

For me personally, it is unethical, then again I have more ideas than I reasonable have time to pursue, so I really don't need someone else's. I would also think of it this way, if word got out that you are stealing ideas then your freelance career is dead.

As for legally, you should consult a lawyer but if it is not in writing, they are going to have a hard time proving that you had a confidential agreement.