The alpha version of Tweetbot has been pulled because of Twitters new rules they have a limited set of tokens available for users during the beta testing phase. Those of you who have downloaded the application can continue to use it during the beta phase.
Paul Haddad from Tapbot writes:
As some of you may have already noticed the download link for the Tweetbot for Mac alpha no longer works. Twitter’s latest API Changes means now we have a large but finite limit on the number of user tokens we can get for Tweetbot for Mac. We’ve been working with Twitter over the last few days to try to work around this limit for the duration of the beta but have been unable to come up with solution that was acceptable to them. Because of this we’ve decided its best for us to pull the alpha.