As of August 31, Twitter is forcing all third-party applications to use OAuth for authentication. That's good news for users (since you don't have your password stored by multiple developers), but it does mean you have to make sure your preferred application is up-to-date.
As Twitter points out on its blog, the switch to only using OAuth has been in train since last December, so if your favourite client is in active development, it should have already made the move. (That's already the case for Twitter's own smartphone apps, as well as TweetDeck, Seesmic and Twitterific.) However, if you are experiencing connection difficulties, check for updates within the application or on its web site.
Twitter applications and OAuth [Twitter Blog]