Permission denied on some users' authorized_keys file with OpenSSH 3.3

[Randall Gellens <randy_(_at_)_pensive_(_dot_)_org>]

Using OpenSSH 3.3,  some users are unable to use public key 
authentication.  The error in /var/log/authlog is: "Authentication 
refused: realpath /home/username/.ssh/authorized_keys failed: 
Permission denied"

I tried changing this user's authorized_keys file to be 
world-readable, and I've tried changing the ownership.  It makes no 
difference.  Here is what the permissions are now:

    -rw-r--r--  1 username  staff  3639 Jun 28 10:34 authorized_keys

I tried creating a new key pair for myself on a different machine, 
and added the public key to this user's authorized_keys file, and I 
get the same error.  If I copy the entry to my authorized_keys file, 
it works.  The permissions on my authorized_keys file look identical.

Can anyone offer any advice?

(Please cc me on any replies: my subscription is set up so that email 
from this list only available to me at home, which is usually what I 
want, but in this case I'd like to see any replies as soon as 
possible.)