This week, the Library of Congress and Google both announced that they are developing digital archives allowing researchers to search all public tweets since Twitter debuted in March 2006. According to their blog, the Library of Congress has acquired historical tweets dating back to March 21, 2006, which will be added to their collection. More details are to come, but they have said to "expect to see an emphasis on the scholarly and research implications of the acquisition."
Google also announced via its blog that the new Google Twitter search will allow users to "replay" tweets by choosing a specific date and even time of day to search for tweets relevant to their search term. To use the new feature, click "Show Options" under the search box, and then "Updates" in the column that will appear in the left. Google has provided an example here. Only the past two months of tweets will be available for now, but Google plans to expand its archives back to March 2006 by this summer.
Bing also has a Twitter search - read more about it in our previous blog entry.