I wrote some mail a few days ago about problems with sendmail,
specifically root not being able to send mail to anyone, but other
users can e-mail each other and can e-mail root also.  Another 
seemingly unrelated problem with users not being able to log on was
actually related.  Anyway, this was the solution e-mailed from
James Burton:

On Mon, 24 Jul 1995, James Burton wrote:

> I've had this crop up a few times and have never been able to figure out what
> caused it (I most certainly am not stupid enough to do it by hand)
>
> Your root directory's permissions are not set up right. The user cannot access
> '/' so the cannot use it to find /bin/csh. Twice I have had to manually set
> permissions on '/'.
>
> I normally have it set...
>
> james@ArtWorks::~ [102] ll -d /
> drwxr-xr-x  17 root  wheel  512 Jun 12 00:13 /

Basically I dont ever remember altering the perms on the root directory,
but re-fixing the perms fixed the problem.