Twitter has changed how you can receive direct messages. Users no longer have to be "following" each other to exchange the private communications. The new system is opt-in only, which allows users to preserve the privacy and quiet state of their inbox, if they prefer.
DMs are more or less Twitter's equivalent of private email conversations or IMs--traditionally they are only available for people who actually follow each other. It's a powerful system that many people use as an alternative to snowed-in email inboxes. But the requirement for users to have the follower connection made things tricky for power tweeps with many thousands of followers. If you now opt-in through Twitter's new settings, you can choose to receive DMs from any user that follows you without having to follow them back.
Twitter's move may be a subtle one, but it does add some very important utility to the system, and positions it even more as a 21st-century alternative to email. The company has also recently adjusted how verified users interact, allowing these Twitter uber-users to filter out non-verified communications if they wish. As the company approaches IPO, this sort of maneuver may be important to show Twitter has larger long-term goals than merely growing its user base--it wants to be used in more creative ways.
[Image: Flickr user Frank Hebbert]