Ahh nice and something I always want in any CMS. I was thinking of this lately and have another point to add:

AJAX is a nice way to throw in dynamic content while keeping most content statically generated and cached. Before I try to explain that think of this : most content on any site with a CMS like Drupal will have content that will become static : like news (once occured does not change in content), or description or almost anything. What becomes dynamic is thus the content in what Drupal calls the “blocks”. This is something that I have seen in almost every site. There is a neat distinction in a webpage’s main content (news article for example) and its dynamic blocks: ads, who’s online, latest poll, etc.

Now since this makes it clear to understand what I am thinking of : cache (static HTML) everything and let the blocks come through AJAX. Now the data for the AJAX blocks that need dynamic data can simply be come from non-cached datasource. This mix will work for almost any type of site, but the module is complex.

Of course your concept is lot simpler for implementation and valuable for many sites, but most people probably use Drupal type CMS to get the dynamic touch.

This is just in a concept form and I haven’t yet thought as deeply about it… but someday …. :)

I am currently working on the GData module as an SoC 2006 participant.
Cheers !!!