The Library of Congress announced they acquired the entire archive of public Twitter activity since its inception in March 2006. Addition of new data to the archive will create a delay of several weeks between its addition and its availability to the public.
Google has also created way to revisit tweets related to historic events called Google Replay. It lets you relive a real time search from specific moments in time, but Google Replay only goes back a few months now, but it will eventually reach back to the very first Tweets.