I'm experiencing a problem in winbind using both a Fedora13 and a RedHat 4.7 machine.  It appears that something causes full_name to become NULL in netsamlogon_cache.  Removing the cache and restarting winbind returns things to normal.  Here's a test case that always seems to cause the problem (finger after slogin):

[root@arges45 ~]# finger testuser
Login: testuser       			Name: Testuser Lastname
Directory: /home/testuser           	Shell: /bin/bash
Last login Fri Nov 19 09:46 (EST) on pts/1 from localhost.localdomain
No mail.
No Plan.
[root@arges45 ~]# slogin testuser@localhost
testuser@localhost's password: 
Last login: Fri Nov 19 09:46:44 2010 from localhost.localdomain
Could not chdir to home directory /home/testuser: No such file or directory
-bash-3.00$ finger
Login     Name               Tty      Idle  Login Time   Office     Office Phone
bhuffman  Brian C. Huffman   pts/0          Nov 19 12:06 (polaris.xmen.eti)
bhuffman  Brian C. Huffman   pts/1    2:16  Nov 19 13:45 (polaris.xmen.eti)
testuser                     pts/2          Nov 19 16:06 (localhost.localdomain)
-bash-3.00$ exit
logout
Connection to localhost closed.
[root@arges45 ~]# finger testuser
Login: testuser       			Name: (null)
Directory: /home/testuser           	Shell: /bin/bash
Last login Fri Nov 19 16:06 (EST) on pts/2 from localhost.localdomain
No mail.
No Plan.
[root@arges45 ~]# rm /var/lib/samba/netsamlogon_cache.tdb 
[root@arges45 ~]# /etc/init.d/winbind restart

Shutting down Winbind services:                            [  OK  ]
Starting Winbind services:                                 [  OK  ]
[root@arges45 ~]# finger testuser
Login: testuser       			Name: Testuser Lastname
Directory: /home/testuser           	Shell: /bin/bash
Last login Fri Nov 19 16:06 (EST) on pts/2 from localhost.localdomain
No mail.
No Plan.