Why does Twitter use a hash and exclamation mark in URLs, and how do they rewrite search URLs?

It’s become the de facto standard that Google has established to ensure consistency and make ajax urls crawlable.

See http://code.google.com/web/ajaxcrawling/docs/getting-started.html

I believe they are using history.pushState. You can do history.back() in the console and it’ll lead you back to the page.