Ok this is getting very interesting.
I'm definitely looking at a dedicated server, with a VM for Zimbra and another for MySQL/Apache2/PHP5
The way OVH manages VMs sounds pretty excellent!
It would be a big advantage for us to have our intranet DB and Zimbra running within the same internal network, since there's some cool Zimlets that I'm planning to integrate.
However we're about to have a rather short-notice office move so I reckon we'll continue with Exchange (urrgh!) for another few months until ZCS6 is released Mar/Apr time
Then this is definitely something we'll have to look at
Thanks for all the info