Intellectual property does not apply to "work for hire" under US copyright laws. In fact a "work for hire" can be such even if you did it gratis, or was never paid under a contract with the client.
I had a 2 year legal battle with a previous employer over this exact same issue. We ended up settling after my attorney advised me that the chances of winning were almost none.
You should understand that ANYTHING you do on the premises of your employer or on your own time away from work, but work related, according to US copyright code, belongs to the employer. The same goes for a client, because you are essentially the employee of the client, so anything done for a client at the direction of a client or for a client even if it was not under direction of the client can be construed as property of the client.
Take it from someone who has already been screwed by that monkey....
