By decoupling urls in the browser from urls in the web app, it's only going to increase the amount of content in the Deep Web that Google can't index. I imagine it'll be difficult to make GoogleBot crawl and index "pages" after large swaths of the web are using this. And once this Javascript feature is in jQuery, then we can expect a huge number of web developers to start using it by default.
I don't know how much Javascript the GoogleBot currently parses and/or executes, but I imagine it would be enormously complex to embed v8 into the crawler, and then execute all Javascript on every page and determine what to index on a "page." If we ever get to the point where you can't screen scrape a web app without also executing its Javascript, then PageRank-like methods become less effective, and we'll need more semantic approaches to search. It's a great thing such semantic approaches don't yet exist!
Some Google crawls already execute Javascript; they could easily do so more extensively if that were the only way to reach valuable content.
Also, Google is already promoting a convention for making the various parts of an AJAX application reachable via different URL-#fragments more easily crawlable. See:
Microsoft for one would love to have a big chunk of Internet real estate that couldn't be found and indexed by Google.
It's funny you mention Facebook. I would say Facebook's chief motive right now is to stop Google from encroaching upon their territory, so Facebook wants to put up as many speed-bumps as possible around Google. Basically that means keeping Google sandboxed—it's okay to let Google index anything they want and throw that up on an ordered SERP, but no way can we let them grow elaborate ivy APIs and web-based UIs around our walled-garden of user generated content. (i.e. Buzz.)
Given Microsoft's ownership and early partnering with Facebook, Microsoft and Facebook have aligned interests in keeping Google in the sandbox.
