I check back in about once every two months to see if there have been any breakthroughs.
I wish the Docker team hadn't more or less given up with boot2docker, since it completely abandons the vagrant advantage of a folder shared with the OS. People do wierd shit like trying to mount their OSX home dir over NFS and, well, Vagrant solved this a long time ago.
If what you say is true, Docker would be a great way to do development through SSH on a VPS with minimal RAM and resources. I'll definitely take a look at it.
VMs are great when you want to test cross-platform or cross-architecture or legacy compatibility all on one machine (there's more to the world than Ubuntu).
While right now, it's a linux only thing, with the incoming support of Windows, things will start to change dramatically.
It isn't a VM though, and there are some isolation issues, but primarily as a target for security, not something that results in different versions or tools being a issue.