I found this issue on custom post types that have featured images enabled, but not the editor. Something like this:
'supports' => array('title', 'thumbnail')
If a featured image is added to one of those posts, it shows up in the media library and other post types can use it as a featured image, but it can't be added to the post. The "Insert into Post" button is missing when you browse to the image to add it.
I ran into this problem with my own plugin Meteor Slides, but the issue isn't specific to my plugin, it can be reproduced with any custom post type.
I used the example in the Codex to test it, and the problem was the same as with my plugin.
Looks like WordPress is setup to remove the insert button when it can't be used due to lack of an editor, but it getting over zealous and doing this on all post types.
Seems like a bug to me, I checked Trac and couldn't find a similar issue. Has anyone else run into this problem or are you able to reproduce it? I wanted to get some feedback before putting in a ticket.