Nambu is a native Mac application designed for multiple social networking services, but in practice it seems like its primary strength is as a Twitter client. It offers support for multiple Twitter accounts, which can in turn be viewed in several different ways. It also integrates support for the tr.im URL shorting service and the pic.im image service.
There's the standard list view which will automatically integrate tweets from multiple accounts into a single timeline. There's a three-pane view, somewhat similar to Mail, with a sidebar allowing you to choose between accounts, and finally (my favorite) there's a multi-column view reminiscent of the Adobe AIR-based TweetDeck and you can, of course, choose what appears in each column: main timeline, replies, sent messages, searches(!), etc.. In addition, replies are automatically threaded so they appear indented under the most recent tweet from the person to whom you are replying (as well as within the normal timeline). Another nice feature is that it automatically displays the domain of shortened URLs that appear in your timeline.
In short, I think I've found my new Mac-native Twitter client. Nambu is a free download from The Nambu Network.