I know I'm showing my age by saying this, but I really don't see the benefit of AJAX in this case. It's basically a frameset powered by javascript, so why not just use a frameset to begin with? It would work in more browsers without issues with back-buttons and take advantage of their built in functionality.

AJAX has its place, but I don't see it here.

Edit: I find it amusing that the documentation for the <frameset> tag is missing :-)