Comment 84 for bug 1197395

(In reply to comment #16)
> Dec 14 14:49:45 f16 su: pam_systemd(su-l:session): Asking logind to create
> session: uid=0 [...]

It got that uid from the loginuid. Calling pam_loginuid.so from /etc/pam.d/su-l would make this work, but I guess the purpose of loginuid is exactly not to be called from su...