By default it's not possible to override existing paths with Page manager, the excellent module that is bundled in the CTools project. The Page manager existing pages module now allows you to do that. Technically, this module defines one abstract task and one content type plugin, so menu items can be overridden and the original page callback can be called through the content type plugin. This project comes with one default existing page, which is 'node', the default Drupal frontpage.
Basically, you are now able to override any Drupal path in your installation and create variants for it. The module comes with one default existing page (although new ones might be added in future). Default contexts for entities is possible as well. I've created a screencast so you can see the module in action at http://www.youtube.com/watch?v=W-4g01WjwI4.
Daniel "dereine" Wehner has written a blog post about it at http://blog.erdfisch.de/2012/01/override-all-existing-pages-panels with some excellent screenshots!