If anybody's wondering about the portable volumes, they're based on Flocker (https://github.com/ClusterHQ/flocker) which uses ZFS to move data around between nodes.
We are also working hard on alternative Flocker backends to support other use cases. For example, we have a block device backend to OpenStack Cinder in the works (and afterwards AWS EBS), which is more suited to high availability than migration - watch this space!
I was going to do the demo that Ben does here but ran out of time to download the VM images, so I'm really glad that Ben did it and that he added Compose support into the mix as well :D