Changes to meta capability handling in 3.1
If you were doing something crazy with custom or meta capabilities and custom post types in 3.0, you’ll probably want to read #14122.
Custom post types had inconsistent meta capability handling in 3.0. current_user_can( ‘edit_post’, $id ) and current_user_can( $post_type->cap->edit_post, $id ) mapped to different capabilities when capability_type = post. We never called the former in core, however, so this would have only affected plugins in a very particular situation.
Now, we have the map_meta_cap flag for CPTs, which allows this to be throttled properly. map_meta_cap is also true by default if capability_type = post or page, and no custom capabilities were defined.
Making this post out of due diligence, but I don’t think this should break any plugins, except in the “This was never supposed to work” situation (which I do want to know about).