It's long been a desire of many Twitter users to have access to their old tweets. Currently, a user can go back only so far in their timeline (or someone else's, for that matter), and the bar for how far back or how many tweets one can access seems to be always moving. Today, Twitter CEO Dick Costolo told Jenna Wortham at The New York Times that the company is currently working on a tool which will allow users to export all of their old tweets, though he didn't have a timeline for its release.
This will likely be good, welcome news to many people who want to have a look at what they were tweeting a few months or even years ago, but the promise of such a feature won't also cover an even more desired feature: the access to other users' tweets. That, Mr. Costolo said, is another development issue entirely. The Library of Congress, which announced back in April of 2010 that it would archive all tweets, has not given access users access to its data, though certain companies, such as Gnip, are authorized sellers of historical Twitter data ranging back to 30 days. While Gnip has worked with the LOC on its archiving project, the Library's data is specifically non-commercial, and will go back, presumably, all the way to the beginning.
The service, which claims 140 million active users, could see some widespread changes in behavior if users are ultimately given access to their own and others' historical tweets. Twitter operates now as a largely "in the moment" service, where users likely feel the freedom to express themselves at any given time, without too much expectation of having to live with the reality of every past tweet forever. There is, however, no indication that that reality is in the near future, so for now, users can continue to tweet every day as if it were their last.