I've come across several integrations between wordpress and symfony, but they're created using the aim of managins static pages content inside wordpress and symfony serving as the controller.
Wordpress wordpress plugin development api appears really fundamental and primitive and you've got not a way o code using the MVC pattern as they are. Therefore the large real question is, an amount be an beginning indicate create plug ins in wordpress, and make admin sections inside it, but using Symfony energy to handle database access, form creation and validation, and so on....
I'd briefly considered an identical method for integrating Lithium with WordPress, but went into a lot of hurdles that made another approach appear more useful: create an MVC framework which was particularly made to work within WordPress.
It's not as full-featured as Symfony, Rails, etc, but WP MVC has similar architecture towards the major MVC frameworks and really should be simple to get if you are acquainted with them. It ties in directly with existing WordPress functionality. Whenever you create scaffold for any resource, for instance, the code that's produced includes an admin controller and admin sights (which look much like WordPress's native index/add/edit sights for posts), along with the appropriate WordPress routing along with a public controller and public index/show sights to show the resource around the public side, inside the active theme.