I have a number of Macs and Red Hat servers running with sshd and kerberos (using an MIT KDC running on Red Hat). I can log onto any of these servers just fine with Tiger, but with Leopard, I cannot. I've set up kerberos on the Leopard client, and I can acquire a ticket. However, when I try to log on to a server, it appears that the client isn't even trying to send the kerberos ticket.
I will not take credit for the following solutions; they come from Apple Support. However, I've tested them both, and they worked for me.
10.5 disables gssapi authentication by default. You have to edit /etc/ssh_config, uncomment the line containing GSSAPIAuthentication and change no to yes. Engineering claims this change was made in 10.4.9 and later, but 10.4.10 and 10.4.11 allow gssapi authentication by default for me.
You can also run ssh server.com -o GSSAPIAuthentication=yes to force GSSAPI authentication.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071128073527767