I'm using a ISPConfig3 cluster based on Debian Lenny and ISPConfig3.
Each server (Xen-VM) is based on your howto
Everything works great, but sooner or later i need to upgrade this whole cluster from Debian Lenny to Debian Squeeze.
Although i'm quiet sure how to do this, about certains things i'm not sure.
Things which are important to know are for example:
upgrade the xen domains (domUs) first or the host (dom0)
Other thing to beware off are written here: http://wiki.debian.org/Xen
I think that many Howtoforge members use this kind of XEN-environment.
A step by step howto would be really helpfull for us all.