Hi,
I've been working on an "extension" to the edit page which allows editors to generate page previews on the fly, directly into the browser and with no need for page reloads.
I've written a draft with instructions on how to install it which you can find at:
http://en.wikipedia.org/wiki/User:Pilaf/Live_Preview
So far it's still in development and it has some limitations, some of which I plan to overcome soon, yet I believe it's mature enough to be useful to some.
Please let me know what you think about it.
-Pedro (User:Pilaf)