With Twitter purging inactive user accounts -- including potentially numerous notable dead former users -- preservation and archival are again concerns.
There are serveral extant public archives:
http://www.archive.org/details/twitter_cikm_2010 (2006-2010)
http://www.archive.org/details/2011-05-calufa-twitter-sql (2011 -- 90 million user scrape)
https://archive.org/search.php?query=twitterstream (Archive Org)
All from the ArchiveTeam page:
https://www.archiveteam.org/index.php?title=Twitter