/#!/ Why is there an exclamation mark in Twitter URLs?

A url hash that starts with an exclamation point is a convention created by Google to allow Ajax web apps to be crawled by search engines.

It indicates to Google (and presumably other crawlers) that the site supports the AJAX crawling scheme. More here:

It's called "shebang" and a discussion was done on SO few weeks back.

Basically the purpose of the shebang in this case is to tell Google how to index pages.