Hash Marks As Ajaxified Permalinks
The hash mark (or the fragment identifier) has become the choice for constructing AJAXified permalinks.
The question mark query string ( For eg. http://google.com/search?q=SomeSearchQuery ) was designed for this purpose. But this forced a reload of the page, not an option for most AJAX apps. The hash mark (fragment identifier) is a link to a part of a document, hence a completely client side action which does not force a reload.
Gmail is the latest AJAX app to go the hash-permalink way.