[odiroot]

Or you could use progressive JavaScript, AFAIK. Make your every link a first class citizen (with it's own view) and for supported browsers replace links' default action with content switching (without reloading) using JS requests.

Of course that's a bit harder since your views need to respond in a different manner to casual GETs and AJAX requests.

[encoderer]

Which really should be trivial using most modern 3-Tier frameworks. They universally have a concept of "layout" vs "template" so in a pseudocode logic, all you need to do is tap into the render method and swap out an empty layout when responding to an XHR request.

[odiroot]

Exactly, however one can argue the number of your test cases (and cache entries) doubles. Still I think it's an elegant solution.