Hi Chris, I don't think that the Docker team have gotten that far yet in knowing where they are going to go, but you can imagine that in the future I blogged about that you might be able to layer OpenShift's gears on top of Docker - now that would be very cool!
There is certainly some convergence going on here which is really exciting for PaaS.
I have to say the Cartridge API does look quite good. Has anyone had any experience with it to share?