Thank you for your excellent Install guide on Ubuntu .I could get Zimbra running on the XEN DomU Ubuntu 6.06 LTS.
:~$ zmcontrol -v
Release 4.0.3_GA_407.UBUNTU6 UBUNTU6 FOSS edition
It was a bit tricky to get the Zimbra installed on a domU because of the NPTL .
Here is what I did to install on Zimbra on Xen DomU
I followed the Instructions to create the Xen Dom0 and the DomU as per the link
http://www.howtoforge.com/xen_3.0_ubuntu_dapper_drake .During the installation of the DomU just
skip the following line
Disable Thread-Local Storage if it has been updated.
mv /lib/tls /lib/tls.disabled
if you do not skip this line the UBUNTU DomU will not Load the NPTL libraries.
After you finish the GuestOS .Install Zimbra as per the instructions on the Begining of this thread