Guys I really don't see this as an exploit. It's more of an annoyance to web programmers, and one that I've been dealing with since IE4.
I've known about this for quite a long time. If there was an exploit that would work, someone would have found it already. But IE actually unloads the old page from memory -- it doesn't "hide" it as the guy who thinks he's someone suggested. Functions in scope create a new document via document.write, and as soon as all the functions go out of scope, a temporary page is generated in-memory and that is loaded up as a new page (note that your Back button is enabled when you go to the proof-of-concept page).
But as soon as the functions go out of scope, the system stops processing data from the old page except anything in document.unload.