I have written a little JavaME client for Twitter, called Zwitscher.
Over the weekend I have published a very first version, that is already able to post tweets and to get the friends timeline. So far it is still at a very basic level.
And I have only tested it on my SonyEriccson K610i phone.
If you are interested, you can download the current version from http://bsd.de/zwitscher/. Please provide feedback.