This problem arises because Docker positions their intrinsics as novel containers, even though there was an entire field of prior art long before Docker showed up. Hence your confusion and worry checkpointing will never be supported; it already was, but you didn't know about it, because to you Docker == containers. One of my many problems with Docker, because they feed that.
Look how easy it is: http://criu.org/LXC
I have to agree though that it seems strange to me that Docker has gotten a lot of support so quickly when things like LXC, FreeBSD Jails, or Solaris Zones have been established for so long. I've only played with Jails a bit, having done more work with full virtual machines in Linux. The other containers, I at least had a familiarity with before Docker (nothing in production). However, I had not heard about CRIU until today. But it looks really interesting!
But, I wouldn't call that a "live migration" in the VM live migration sense.
However, I wouldn't worry too much about it, since you're absolutely right that most processes can restart gracefully.
I'm not sure this would even be possible outside of a virtual machine. VM hosts have significantly more information and control about the VM than the container host has about the container. For example, a VM host knows exactly what memory is hot and what is cold to enable sub one-second switchover times.
It might be possible to add such support to the Linux kernel itself to support process migration between hosts, but that's the level of work required.
The alternative that I was thinking about using your standard high-availability tools to create a new worker on the new host, then gradually remove existing workers on the old node. I think that might be the only way to really make "live" migrations work from a practical standpoint. For web-like services, this would work. For others, it may not be practical.
EDIT: Looks like the checkpointing running processes might work after all! (see: http://en.wikipedia.org/wiki/CRIU) I must admit that I'm still a little skeptical, but would love to see this working!