I have a version of xdm that I patched to do this. You can get it at 
ftp://ftp.styx.org/users/ww/xdm-krb5.tar.gz. It will authenticate with
kerberos and give you tickets that are forwardable and renewable.

You'll need to put something like the following in
/usr/X11R6/lib/X11/config/site.def before you run xmkmf.

#define HasNKrb5 YES
#define NKrb5Includes -I/usr/krb5/include
#define NKrb5Libraries -L/usr/krb5/lib

I have had this build correctly under NetBSD 1.5, Solaris 2.6 and
HP-UX 10.01.

X11R6 has some kerberos code in it -- in fact it may at one time have
been possible to grant access to your display via kerberos
tickets. It was written with an old beta of krb5 and no longer
compiles. So, even though '#define HasKrb5 YES' in your X confguration
appears as though it would do something usefull, it won't.

Let me know how it works out.

Cheers,
-w
____________
Will Waites \________________________
Minister of Research and Development \____________
Idiosyntactix Strategic Arts and Science Alliance \