“Twitter Blackbird Pie is a method of displaying tweets as rich full content rather than as just simple URLs or images. With Blackbird Pie Twitter, hashtags link to search pages and usernames link to twitter profiles. An individual tweet, or pie, includes all the details, design, and information that a single tweet page would include.”
Twitter Blackbird Pie will work for posts and comments throughout WordPress.com. For self-hosted WordPress blogs there is a plugin: Twitter Blackbird Pie plugin.
More info: http://en.blog.wordpress.com/2010/11/05/bring-tweets-to-your-content/