Like many others, I've today run into the problem whereby the 'add media' and 'featured image' buttons have stopped working with 3.5 upgrade.
This is confirmed not a WP bug, it is 100% a theme compatibility issue with the theme I am currently using, and sadly it looks as though they are slow to fix theme bugs :-(.
The issue seems to be a JS conflict between the new WP add media button and (I think) the theme's use of underscore.js to invoke the old media uploader in their custom theme option fields - e.g. upload logos/thumbnails etc.
I'm wondering, seeing as their custom theme fields seem to still be able to invoke the old media uploader, could there possibly be a quick and dirty way to force WP to use the old uploader for the time-being while the developers of various plugins/themes fix their products? The uploader still seems to be present in the codebase in some shape or form.