1. A user interface extension method for a componentized application environment, the method comprising: specifying in markup a set of user interface controls; identifying within a manifest markup for a set of extensions to corresponding ones of the user interface controls in the set of user interface controls; substituting markup for each of the corresponding ones of the user interface controls specified in the markup with markup for an associated one of the extensions identified within the manifest; and, providing markup for each of the extensions and the corresponding ones of the user interface controls in the set to corresponding control factories without also providing a substituted user interface control for each of the corresponding ones of the user interface controls to the corresponding control factories, the markup being provided for generation into user interface control instances in a data processing system including a componentized application environment supporting one or more plug-ins to the environment, the user interface control instances defining a user interface for the componentized application environment.