Possible Duplicate:
What's the shebang (#!) in Facebook and new Twitter URLs for?
What does the (#!) in Twitter URLs do?
Possible Duplicate:
What's the shebang (#!) in Facebook and new Twitter URLs for?
What does the (#!) in Twitter URLs do?
Its for search bots: http://code.google.com/web/ajaxcrawling/docs/getting-started.html
It's called a shebang or hashbang, besides the above mentioned (making Ajax actions crawable). It does, in a more general sense, make Ajax actions and results more accessible, for example, at Facebook. For example the URL http://twitter.com/#!/search/stackoverflow redirects to http://twitter.com/search/stackoverflow - so you can just give it to someone and that person sees the same as you did after invoking some Ajax functionality. Facebook does the same with picture galleries.