If anyone gets that done it will make us very happy! We run our servers on CoreOS, putting it on the devices as well would be awesome. AFAICT it's about getting etcd to run on the Pi, as well as any other small projects that make up CoreOS.
There is an ongoing project to bundle Docker with Tiny core linux, which is even more lightweight than coreos. Meanwhile there is also work to add clustering capabilities to docker.
I looked for something like this myself - the closest I think you could get for now is a bespoke, stripped-down Gentoo with Docker. That said, there's a conversation taking place on the CoreOS mailing list that looks promising (https://groups.google.com/forum/#!topic/coreos-dev/cioNy0GjC...).
I haven't seen anything yet, but I'm very interested in this area – I've got an application that uses a bunch of RasPi nodes, and running anything like Raspbian or even PiLFS feels too heavyweight for what I ultimately want (a single networked app that runs on startup).
Anyone have any ideas?