intergrating a MVC (php) Framework with a CMS

I do something similar with Zend Framework & Wordpress. I basically do a regular Wordpress install which I set as not visible and symlink my wordpress folder to the ZF library folder, this makes all the Wordpress content & functions accessible within my ZF application.

This might be frowned upon since I'm including all sorts of stuff that really is not needed... but it works well and as easy to upgrade as upgrading Wordpress itself.

Joomla is MVC out of the box, seems like it would be the easy solution since its designed to be used exactly as you are describing. 1.6 is in the later stages of beta and includes quite a few access level enhancements that would also be useful.

We'll use symfony + drupal to a similar project, but now it's just a PoC and you can integrate both way : Symfony app inside a drupal node or drupal layout as a drop-in replacement for symfony layout.

An alternative I suggested was to use a Symfony based open source CMS/CMF (like Diem) to have a symfony only environment which would ease maintenance and initial development.