After applying the patch reference Ticket ID: 41385 "Visibility of custom field on registration is incorrectly controlled by setting "display" (fix completed in svn)", I find that submitting a registration that has "Display profile field" set to "no", results in the data for that field not being stored.

Please see my reply to http://www.phpbb.com/community/viewtopic.php?p=9435505#p9435505