Added ManTweet_20080624 which has the following fixes:

1. Trim strings before posting them.
2. If string is blank, trigger an error, rather than add an empty post.
3. Add a configuration page for the plugin.
4. If the logged in user doesn't have access, then hide the menu option.
5. Added a configuration threshold that specifies what access level is required to publish updates to the public Twitter account (default NOBODY).

By the way, the plugin source code is hosted at the following location:
http://github.com/vboctor/mantis-plugins/tree/master

As reported by paulr, there was a Javascript error in ManTweet. For any user that did not have access to submit a tweet, the auto-focus script attemted to execute the focus() method of a non-existent DOM object. This was fixed in Git by wrapping the auto-focus in a conditional.

See revision 9fcc7d: http://github.com/vboctor/mantis-plugins/commit/9fcc7d70e768bc0fc8d413569968d6cc5a41899b

Edit: the JS error would not show up in Firefox, but I was able to replicate it in IE when not logged in and visiting http://bugs.mantisbt.org

Thanks jreese. I've updated ManTweet on this bug tracker with your fix. Thanks.