10 Ways to Archive Your Tweets

"Did you know that your tweets have an expiration date on them? While they never really disappear from your own Twitter stream, they become unsearchable in only a matter of days. At first, Twitter held onto your tweets for around a month, but as the service grew more popular, this "date limit" has dramatically shortened"

http://www.readwriteweb.com/archives/10_ways_to_archive_your_tweets.php