The trend of using Twitter for absolutely everything doesn't show signs of stopping.
Tweetboard, the latest hot Twitter add-on, inserts Twitter as a comment system for your website. Conversation on the site is posted (neatly threaded, of course) in an expandable sidebar, and a user's comments also post to their Twitter account. To make it easier for people who are reading these tweets outside of Tweetboard, there are two shorturls "posted.at" and "inreply.to," that give readers a little context and a link to view the thread.
Tweetboard's design is relatively inoffensive: it puts a tab off to the left side of your site with the number of tweets a site visitor hasn't read. Fortunately, it's a pretty small tab, so it's not completely annoying to people who don't care about Tweetboard. Once expanded, Tweetboard is laid out with tabs for all comments or just the current thread, and there's a space at the top to post your own comments.
Judging by the Tweetboard's quick jump to the top of Twitter Trending Topics when it launched, you're probably going to start seeing it all over the place pretty quickly. It's hard to tell right now whether it's going to be a momentary fad or a lasting fixture. I can see the quick setup and ease-of-use appealing to site owners, but users might not go for Tweetboard. Some people just don't want Twitter accounts, and some people who have Twitter accounts might prefer that you keep your comments on your own site.