A prototype version of bulk editing now exists in the Backbone branch.
The sidebar uses a view media.view.SelectionBulkEdit, which directly augments media.view.Selection, adding support for the bulk delete button. See the full commit for the magic.
Selecting a single image brings up the attachment details sidebar as per usual.
Selecting multiple images (with cmd+click or shift+click) brings up a bulk edit view in the sidebar.
Clicking delete will bring up a familiar confirm dialog before taking action.
The images are immediately deleted from the library.