Let me explain the proposal in a very simplistic way, that honestly over simples it, but does give non-technical people an idea on what this does.
Here is the technical overview by Google:
Starting with a stateful URL such as http://example.com/dictionary.html#AJAX , it could be available to both crawlers and users as http://example.com/dictionary.html#!AJAX which could be crawled as http://example.com/dictionary.html?_escaped_fragment_=AJAX which in turn would be shown to users and accessed as http://example.com/dictionary.html#!AJAX
For a more detailed look at how this works, see the blog post. Of course, servers and system admins would need to make this possible and web developers who have to code this in.
Wow - if this gains enough of a following, it could really open some new doors to creating rich interactivity. We're back to the roots of the problem here, which is that the 'single content, single url' model, coupled with browser technology and the internet in general were never designed to support the levels of interactivity we're pushing down the tubes.
I don't care for it, for several reasons.
Beginning with the fact that they're proposing introducing yet another illegal character into url strings and ending with it sounds like a lot more work than simply creating accessible Ajax from the get go.
My bottom line is simple. If it's important that a site or an application use Ajax and if it's also important that the site be accessible by bots and real users alike who suffer from a disability, then it should be important enough to build your Ajax app to be accessible. If not, don't.
As one of the comments pointed out, to me it looks like a complex, very questionable solution in search of a problem.
Unfortunately there is now a lot of inappropriate ajax around the web - the kind of thing that's done mostly just to display someone's technical prowess (geek credentials.) That approach is hiding useful content and I think such situations are what this proposal is an attempt to resolve.
As you can imagine, there is the possibility of cloaking here. That means, I can show a user one thing on the first URL and the search engine another piece of content on their URL. Google would have to somehow validate all of this, some way.