There's a good article by Ecomdev that gives an example of how you can do this with the 'depends' functionality. Search for 'depends' on that page and it will demonstrate how to use a config value. e.g.

    <depends>
        <config>payment/checkmo/active</config>
    </depends>

The menu item will only show if the check payment method is active.

http://www.ecomdev.org/2010/10/28/defining-acl-resources-custom-and-admin-menu-in-magento.html