Today the Publicize feature gets a major addition: the ability to send your posts to Twitter. You can go with the default tweet or customize the message, and override the automatic settings on a per post basis as well.
To enable the new feature, head over to your Dashboard: My Blogs admin page. In the Publicize column, check off the Twitter column for each of the blogs you'd like to enable to auto-tweet. You'll be asked to authenticate each blog via OAuth, after which you'll see a "Publicize" section at the bottom of the Publish box each time you write a new post.
To override any of the settings for an individual post, just click the Edit link in the Publicize section. You can edit the text of the tweet or turn it off altogether for that post. Note also that the connection settings are per blog and per user, so if you run a multi-author blog each individual writer will have to authenticate to their own Twitter account (or to whatever account is preferred).