Twitter does this: http://twitter.com/#!/twitter is parsed by the Google crawler as: http://twitter.com/?_escaped_fragment_=/twitter which is a 301 Move

Twitter does this: http://twitter.com/#!/twitter is parsed by the Google crawler as: http://twitter.com/?_escaped_fragment_=/twitter which is a 301 Moved Permanently to the old: http://twitter.com/twitter which in turn redirect to the #! version if you are logged. This is a mixed approach that mantain backward compatibility (and support Bing, Yahoo and other crawlers).