>I signed an NDA at a client's request
>
>Now I want to apply for a job opening in the client's company. 

You signed an NDA for a client, now you want to know if you can tell that client you signed their NDA? 

Seeing as how your agreement is with them, I don't see how telling them about your agreement with them would violate that agreement. Perhaps you should tell them about your concern, and ask them if they'll release you from the NDA for the purposes of talking to them about it and applying to work with them.

Nevertheless (to cover myself here), this is getting into contract law territory - you should probably ask a lawyer. If you have the resources to consult with a lawyer, it's probably worth it to protect those resources.