yes, support exists today, its just not complete, i.e. its close but not completely feature parity with docker as the runtime. When it does get there we will all be happy.
So I should have been more clear, you can specify rkt today, but many things wont work, a lot of things have improved for the upcoming 1.2 k8s release but still not perfect. hence what I meant by "released", something that is can be viewed as a complete replacement for the docker runtime.
I don't have a list handy but can throw an example
a simple thing that was missing was managing /etc/resolv.conf (see rkt 1.0 release notes that this was added so now kubernetes can take advantage of it going forward).
Without it, one either has to jump through some hoops (not impossible) to manage it yourself or your system just wont work (i.e. trying using GCE metadata server from within GCE without being setup for GCE's dns server).
So I should have been more clear, you can specify rkt today, but many things wont work, a lot of things have improved for the upcoming 1.2 k8s release but still not perfect. hence what I meant by "released", something that is can be viewed as a complete replacement for the docker runtime.