Great plugin. Just wanted to point out a conflict I found with this plugin and WP eMember (http://wp.me/pmEqq-rw). I'm not sure if the problem lies with Simple Fields or eMember, but thought I should post this here in case it is a Simple Fields problem.
The conflict appears to be with the 'jquery-ui-effects-core' script which is queued in simple_fields_admin_init(). With the current version of the plugin, it was queuing version 1.7.3 of this script. This prevented the content protection options in WP eMember from working correctly. If I change Simple Fields to use the latest version of this jQuery file, it works fine.
So to fix the conflict, I changed line 65 in simple_fields.php to: