This plugin is great, however the author has not updated it in over 1200 days! But it seems to work with the current version of wordpress (of which right now is 3.2). But the problem comes in how it looks on the posts. There is no bounding box. So to fix that you need to change a few CSS classes in the auto_tweet.php page. Here's what to change:
Change line 82 class to "postbox" and line 84 div class to "inside"
Then it will look good in 3.2
Author can you please update this so we don't have to do this everytime we use it on a site. Thanks!