Yes, I'm talking about the free version: VMware Server.
I just posted in another thread on this but if you install Zimbra in a VMware Server virtual machine:
1. You can do live, running "backups" of the whole virtual machine using VMware Server's "snapshot" feature. The Zimbra vm can be running during this process. We tested two restores of snapshots, and Zimbra ran fine. But, we didn't have a lot of data in Zimbra at the time, so I don't feel confident saying that this backup strategy is guaranteed to keep ldap, mysql, cyrus, etc. totally in sync. Plus, the snapshots are effectively native to VMware Server, so you can't move them offsite.
2. You can also use the VMware Server "Suspend" feature to freeze the entire Zimbra vm, then copy the virtual machine files to wherever you like. If you don't have a lot of big mailboxes, this is only a few gigs, so your Zimbra machine is effectively offline for not a long time. If you copy the vm file locally, then resume the Zimbra vm, then copy the backup files off site, you will have a totally consistent off site backup, with minimal downtime.
We were also getting ready to test backups using some xfs and lvm utilities, but once we told our Windows clients that Zimbra had an Outlook connector (and they were willing to pay for it), we just upgraded to the Network Edition.
I think the optimal way to backup the OSS version of Zimbra while Zimbra is running will be to use the xfs filesystem, and/or LVM partitions, both of which have a "freeze" or "snapshot" capability. But both xfs and LVM are very kernel dependent, and while both LVM and xfs are enterprise-grade, their snapshotting capabilities seem to have some kinks still. As a result, if you really need support for backups, I'd pony up for the Network Edition.
If you can afford the time to experiment, I'd install Zimbra on two machines: one with an xfs filesystem, and one with ext3-formatted LVM partitions. And then I'd test which one performed backups and restores more reliably with your chosen distro.
The wildcard here is Xen. We initially tried to deploy Zimbra in a Xen vm, but all our Xen hosts (SuSE Linux Enterprise Server 10 64-bit) only do paravirtualization and so won't support 32-bit Xen virtual machines. Longer-term, I expect running Zimbra OSS in a Xen guest will make live, whole system backups the most reliable.
Hope that helps,