After much faffing over the past few days I've just completed a successful installation of 4.5.0 BETA RHEL 64bit version on a Xen virtual server (1 dedicated core @3ghtz, 1gb dedicated RAM) ... and it's great, high speed, minimal swap.
The faffing was the custom 64bit kernel for Xen rather than zimbra, absolutely no fuss with the zimbra install. There's a minor bug about adding zimlets (error code 14) which I've not looked into, but everything else runs well (and very quickly)... and it's the beta so I'm not complaining
The dom0 (base) is Ubuntu Edgy (6.10) 64bit and the zimbra domU (VPS) is CentOS 4.4 64bit. Just reporting that it's easily possible with a little patience - remote installing CentOS is the fun bit TIP: always use LVM for the Xen client domains Zimbra's high I/O and the general crapness of disk images would be suicide.
As a random note, now Xen is getting certified by Redhat and other major providers, might be worth 'officialising' it? Can't be a lot of hassle I'd have thought providing you slap a giant warning about it needing a lot of the host server, but if you have the need for say a 25 user Zimbra server and have a quad xeon box with 8b of ram kicking about it would be slight overkill, which is where Xen comes in handy with its tiny overhead.
I've given some friends test accounts and will try hammering it a bit during the week. I'll report any feedback if anything wobbles