It's About The Hashbangs

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).