First I want to say thank you to all the contributors of this thread and msghaleb in particuliar.
I installed zimbra 5.0.15 and Samba on separate Ubuntu 6.06 VMware guests. Everything went fine, except towards the end : my samba domain name would not appear under Samba in the Zimbra admin gui.
To solve, I just needed to had the zimbra server to the /etc/hosts file of the samba server. Sometimes, we forget about the basics...
Cheers