The best way to do this would be to use our API calls for white labels. This will allow you to retrieve a list of all of your client's API keys.
To do this, take a look at our White Label API documentation under http://code.google.com/p/sendible-api/.
This call will allow you to retrieve all of your clients as the admin user:
http://sendible.com/api/v1/profiles.xml (see http://code.google.com/p/sendible-api/wiki/GetProfiles)