WordPress 3.5 introduced a new feature - when images are inserted into posts, the title attribute is not included in the image tag.
Unfortunately, this causes problems for some Lightbox plugins, quite apart from stopping image tooltips from appearing.
This plugin hooks into the media_send_to_editor filter and inserts the image title into the html inserted into the post.
It's released under a GPL licence and is available from the WordPress Plugin Directory at http://wordpress.org/extend/plugins/restore-image-title/.