A new extension has been released for System Center 2012 Virtual Machine Manager (VMM) that allows you to import and export virtual appliances using the Open Virtualisation Format (OVF). OVF is a vendor neutral format for out-of-band migration of virtual appliance VMs. This is the perfect way to get from one cloud to another, where specialisations would make direct import/export impossible.
There is more on this tool on TechNet
You can learn more about OVF on Wikipedia.