User Experience Trumps Pretty URLs, The End.

The text after the # is a Fragment identifier. Wikipedia: "In the most common case, the agent scrolls a Web page down to the anchor element which has an attribute string equal to the fragment value.". Basically, fragment identifiers are commonly used to link to specific content in long documents.

More recently, however, ajax applications have started using them to push state into the browser's history. This enables the expected functionality for bookmarking and the back button. Since fragment identifiers are only used by the client you can link to them without refreshing the page. Using the ! in #! is a convention proposed by Google for stateful ajax applications.

http://en.wikipedia.org/wiki/Fragment_identifier

http://googlewebmastercentral.blogspot.com/2009/10/proposal-...