I am using s2member v3.1.2, and I think I've found a small bug. The S2MEMBER_CURRENT_USER_FIELDS constant does not contain my custom fields.
I have found the cause, in s2member/includes/functions/constants.inc.php, line 211. It uses $result where it should use $r. When I changed that the problem was fixed.
I hope this helps someone.
cheers
David