at the time of writing this post, a wordpress site was upgraded to V3.6 and qtranslate plugin was broken. when editing a page or post, the editor does not allow editing the languages and there's a message displayed above the editor:
The qTranslate Editor has disabled itself because it hasn't been tested with your Wordpress version yet. This is done to prevent Wordpress from malfunctioning. You can reenable it by clicking here (may cause data loss! Use at own risk!). To remove this message permanently, please update qTranslate to the corresponding version.
there is a temporary solution while waiting for the plugin to update.
go to the plugin directory and edit qtranslate.php, locate 'QT_SUPPORTED_WP_VERSION' and change the version number the the current wordpress version.