A little context...
One of the big downfalls of most CMS's in my opinion are tight integration of their presentation and back-end content management layers. I see Drupal, for instance, as tightly integrated. Non "free" CMS's like Alfresco can be used to just create content while skipping any front-end presentation layer implementation in the CMS itself (One could opt to use Drupal in this was as well).
My question...
What I really want, is an open source CMS designed to do just content management, and not care or dictate how my presentation layer should be built. I want a CMS that manages, i18n, workflows, sandboxes, etc. while allowing me to build X number of presentation layers that are ignorant of what and how the content was generated, but consume it any way they desire. One could have the CMS in PHP but the presentation layer in RoR.