I noticed this entry in the Xen knowledge base about OVA:
XenSource Knowledge Base : Can you convert a virtual machine from Open Source Xen to your products?
There is currently no automated mechanism to move VMs from open source Xen to the XenSource commercial products. The upcoming OVA (open virtual appliance) format will allow VMs to be moved between different Xen environments.
OVA looks like it might be a useful way to define generic portable Xen images. Enomalism have some details about this as well. Other than that the only google stuff seems to be from June last year. Enomalism also have a link to this clever idea: rss to push vm images.