A user had accidentally deleted and re-created /tmp but had not restored 
SELinux permission. So while it was mode 1777 it lacked the SELinux 
context so was not writable.

The error from Kerberos when trying to obtain a ticket in mod_auth_kerb is:

[Tue May 01 09:22:07 2012] [error] [client 10.16.186.52] gss_acquire_cred()
failed: Unspecified GSS failure.  Minor code may provide more 
information (, )

I have no insight into why the failure is unspecified but it would be 
helpful to include more information, even if just the ccache location 
being used.

rob