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!