I can't reproduce this.

$ rpm -q samba  
samba-3.6.0-alt1

> All the options are greyed out.

There should be some additional message if something goes wrong.
Are you sure that smb and nmb services is up?

> Also, if I use net usershare add/delete, it works fine.

Very strange, thunar's plugin just uses the same commands, there are no any other magic.

[alecs@rainsoul ~]$ rpm -qa | grep -E 'samba-3|thunar-shares'
thunar-shares-plugin-0.2.1-alt1.git20110411
samba-3.6.0-alt1
[alecs@rainsoul ~]$ groups 
alecs wheel fuse vboxusers hashman alecs_a alecs_b sambashare
[alecs@rainsoul ~]$ net usershare list '*'
torrents
muu
current2
movies
dan

It says: You may need to install samba, check your user permissions (usershare group) and re-login.
I also tried giving 777 to /var/lib/samba/usershares 
Also, with nautilus-share it works.


SOLUTION:
[alecs@rainsoul ~]$ net conf list
Failed to initialize the registry: WERR_ACCESS_DENIED
[root@rainsoul alecs]# ls -la /var/lib/samba/registry.tdb 
-rw------- 1 root root 81920 Sep  1 17:13 /var/lib/samba/registry.tdb
[root@rainsoul alecs]# chgrp sambashare /var/lib/samba/registry.tdb
[root@rainsoul alecs]# chmod 660 /var/lib/samba/registry.tdb
[root@rainsoul alecs]# ls -la /var/lib/samba/registry.tdb 
-rw-rw---- 1 root sambashare 81920 Sep  1 17:13 /var/lib/samba/registry.tdb

Works for me now.