I just found a conflict between the And the Winner Is ... and Members plugins (http://wordpress.org/extend/plugins/members/). Specifically if AtWI is activated, you can't add new capabilities for users, and the existing capabilities are truncated. According to the Members plugin author Justin Tadlock in his support forum:
add_action('admin_init', array(&$this, 'AdminInit'));
It should be hooked to
admin_enqueue_scripts and the
$hook_suffix parameter should be used to determine which page is currently being viewed.
Or, just tell him to shoot me an email."
Greek to me, but I'm passing the word along!