WordPress.org

1.6.1: 2009-09-27: Overdue bugfix release.
* Bugfix: Automatic tweeting now works on scheduled posts! In 1.6,
  it would only work on posts set to "publish immediately."
* Bugfix: Double quotes work in "Send to Twitter" box on Write Post screen.
* Bugfix: Single and double quotes now work in Twitter "Editable text box."

1.6: 2009-08-24: Version 1.6 makes these changes:
* Feature: Automatic tweeting of new posts to Twitter.
* Feature: Can specify custom URL shortening service.
* Feature: Clickable radio buttons in options menu.
* Feature: Better documentation and screenshots.
* Bugfix: Since 1.2: The cached URL for a post would be deleted
  when a comment was added. Now it remains.
* Bugfix: Since 1.2: Tweets would get cut off around 125 characters
  if titles were too long. Now they can go all the way to 140.
* Bugfix: Since 1.4: When link text for Twitter was not [BLANK] but
  link text for another social network was [BLANK],
  "[BLANK]" would be displayed. Now no text is shown.
* Bugfix: Since 1.5: Pressing enter in a text field in the options
  form would trigger "Import Options" button. Now it triggers
  "Save Options" as it should, because the Import / Export
  section has been moved down.
* Bugfix: Since 1.5.3: When Reddit icon was changed but link text
  was left as "Reddit", it would not get changed to "[BLANK]".
* Removal: Qr.cx, ri.ms, hex.io, lin.cr, idek.net, urlb.at, and
  h3o.de have been removed from the URL shortening services.
* Removal: The buggy "Tweets about this post" RSS feature is gone
  (previously only accessible by tt-config.php option).
* Removal: WordPress 1.5 - 2.2.X no longer supported.

Older versions: http://richardxthripp.thripp.com/tweet-this-version-history