This bug was originally reported here:
  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=635168

Some Debian powerpc users have noticed accountsservice prevents them to see
more than one user (the one with the highest UID) in gdm3 login window and
complain that they have to select "Other" and manually type the username
they want to login as.

This issue is specific to the powerpc architecture and the following patch
seems to be a good fix:

diff -up -ru accountsservice-0.6.15.orig/src/user.c accountsservice-0.6.15/src/user.c
--- accountsservice-0.6.15.orig/src/user.c      2012-02-07 10:09:11.000000000 +0100
+++ accountsservice-0.6.15/src/user.c   2012-02-07 09:04:53.000000000 +0100
@@ -731,7 +731,7 @@ compute_object_path (User *user)
 {
        gchar *object_path;

-        object_path = g_strdup_printf ("/org/freedesktop/Accounts/User%ld",
+        object_path = g_strdup_printf ("/org/freedesktop/Accounts/User%lld",
                                       (gint64) user->uid);

        return object_path;