Thanks. Indeed this is working fine (I've implemented it on Ubuntu server 9.10 with ISPConfig 2.2.23). However, I have noticed that the /./ thing is somehow 'returning' to the /etc/passwd (after I have manually removed it). I suspect that it has to do with the update mechanism of the ISPConfig.
Do you know the reason for that? Is there some kind of a master file for the /etc/passwd , like for example the named.bind.master?