@Sallie Goetsch (rhymes with sketch) - GPL only kicks in if you or your client distribute the code beyond your client. For instance, if your client's business sold the theme to another business, the GPL would give that other business the right to distribute the code too, if they so wished.

The selling point is that, with Open code, your clients are freer too, they know that they are getting code from you that can be viewed and modified by anyone, even if you get hit by a bus.