Twitter's search function is great for finding out what is happening at this very moment. It's so great that Google and Bing have added the results to their search results and, in the process, given Twitter its first revenue source (more $ are coming).
Today, Google announced that it would be searching across the whole Twitter archive.
Starting today, you can zoom to any point in time and replay what people were saying publicly about a topic on Twitter. To try it out, click Show options on the search results page, then select Updates. The first page will show you the familiar latest and greatest short-form updates from a comprehensive set of sources, but now theres a new chart at the top. In that chart, you can select the year, month or day, or click any point to view the tweets from that specific time period.
It will probably start later today because, as of this writing the service isn't available.
Here's a screenshot from Google:
Google isn't the only one getting in on the "Archiving all Tweets" act. The Library of Congress has even announced (Tweeted no less) that they will be indexing "ALL public tweets, ever, since March 2006!"