It's a small new feature on the surface, but it opens up a lot more commenting opportunities for WordPress.com blogs. Third-party commenting systems such as Disqus and Echo have allowed users to authenticate with their Twitter or Facebook accounts for quite some time, but this is a big move for the hosted blogging platform.
In the blog entry announcing the new feature, WordPress.com's Scott Berkun points out a nice feature of the new login system: Users can stay logged in to multiple services at the same time. This is especially handy for users that might want to comment using Twitter or Facebook on some sites but want to use their WordPress.com account for others.
Comments from Twitter or Facebook accounts aren't pumped back into those social services, though. Instead, the logins simply act as a way to authenticate users. In the future, WordPress.com might work toward also giving users the option to publish or share comments back on Facebook or Twitter.