There are some subtle changes on the horizon for the global administrator privileges in Confluence 2.7:
The released milestone contains a new global permission. Essentially we've split the previously existing Administer Confluence permissions into Confluence Administrator and System Administrator. Confluence Administrator is a lesser administrative that provides restricted administrative powers. Eg they have the power to add and remove users, but no power to insert arbitrary code into Confluence by installing plugins.
Plugin developers may need to update their permission checking to allow or restrict Confluence Administrators to perform various tasks.
For more information, see the Atlassian forums.