The new version of Blogo, a Mac desktop blogging application, have native support for Twitter, including a Microblog Viewer to read your tweets, as well as simple posting to your Twitter account. This version is in beta now, but I can attest that at least the Twitter functionality seems to be working as it should. The release is imminent.
Blogo isn’t the first piece of blogging software to support Twitter, but as far as I know others, like Ecto, require you to install addons, scripts, extensions, or whatever they support.
Possibly related: My Devlounge post on Mac desktop blogging applications