I can't say if this will work for anything other that XenServer 5 (w/ hotfix 2) VMs, but I was able to convert them to ESX 3.5 VMs via the VI Client 2.5 Consolidation utility, which I believe is just the intergrated version of P2V Converter.
As a matter of fact, that VM I converted from a XenServer VM to an ESX VM, started out as a VMware Virtual Server 2.0 VM. We mainly use VMware products around here, but began testing XenServer when they released XenServer 5.0 for free. We have all of our DEV VMs running on VMware Virtual Server 2.0 hosts and are looking for a new product.
Initially we were looking at ESXi, but when we looked at what XenServer 5.0 had to offer, the decision was pretty simple. XenServer has a lot of the features that our ESX Enterprise environment has minus the $20K price tag (price based on the number of hosts of course). We've only been testing XenServer out for a couple of weeks, but so far it's pretty impressive. I'm not sure if we would give up our PROD VM envrionment (ESX 3.5 Enterprise) just yet, but I think we are definately going to be replacing Virtual Server 2.0, which we've had problems with anyway.
To convert from a VMware Virtual Server VM to a XenServer VM I used the V2XVA.exe utility from Citrix. Follow these instructions and you shouldn't have any issues:
http://www.ervik.as/index.php?option=com_content&view=article&id=1397:how-to-convert-vmware-virtual-machines-to-xenserver-virtual-machines&catid=13:xen-server&Itemid=34
To convert from XenServer 5.0 to ESX 3.5, just use the P2V Converter (treat the Xen VM as a physical server) or the Consolidation wizard built into the VI Client 2.5.
Good luck and I hope this helps.
