Server side rendering is on the Meteor roadmap, but is probably not going to be implemented soon. Kadira wrote a blogpost about it here in which he managed to get SSR working with FlowRouter (React only). This means that it's technically possible to build Isomorphic apps.
As you may have guessed: Kadira is now using SSR to create isomorphic apps. You can see it in action at kadira.io. Other than that I am not aware of other companies using Meteor to build Isomorphic apps with.