| Yes and no.
If you're trying to typo squat, then definitely.
But if you're making a service that complements and gives something of value to Twitter users, I doubt you'll get in any trouble. Remember that Twitter has made its API public so that others can make twitter add ons. Some of the most popular Twitter services use either Twitter, or Tweet in their names (TweetDeck is an example)
Its always good to shoot them an email though, if just for the sake of records |