There is now a Windows and Mac OS X installer hotfix for the recent Zope security issue detailed here. It is available in the Plone Hotfix release page. Users of the Plone Windows installer are encouraged to install this important security fix (which should apply to Windows installer created Plone instances for all 2.x versions). Thanks to Sidnei da Silva and Enfold Systems for their quick response.
Additionally, the OS X and "Universal UNIX" installers for Plone 2.5 have been updated by Kamal Gill to include a patched Zope version. An OS X installer to apply the hotfix to existing installer-created instances is now available as well.