I created a simple demo last night, you may take a look. For example, this page, when you you open it, it's already rendered on the server:
http://recorder.mvc-works.org/guide.html
But as you click on the entries on the sidebar, it renders without page refreshing. And if you reload the page with "Command+r", you will get a page from server that's already rendered into HTML.
So it renders on both sides, on the server and in browsers. That's a real example for "isomorphic".
In order to render the same HTML on both sides, and with a same copy of the code, you have to make the code decoupled enough from the environments. Otherwise the code throws errors for reason like document is not defined or something. And React is just good at it.
Jon
ClojureScript Developer.