An article at Ajaxian has detailed how you can add Javascript to your web pages to detect if the person visiting the page is logged on to Twitter. While this does raise all sorts of security-related issues and concerns, it seems like you could use this to provide a better experience for users on your own site. For example, just like lots of sites have buttons to add your page to Digg, reddit, or other sites. You could create a button that lets people push it and instantly publish a tweet that they are reading your site... among other things.