Twitter is changing the way direct messages work. In the past, Twitter required two people to follow each other for direct messages to be exchanged in either direction; now, users have the option to change settings so that any of their followers can send them a direct message, without having to follow them back.
The new setting appears to have started rolling out this week. In user settings, there's a now a check box called "Receive direct messages from any user." Next to the button it explains that "[i]f you check this option, any Twitter user that follows you will be able to send you a DM, regardless of whether you decide to follow them back."
It's not the dramatic messaging changes it could've been, and chances are it's really aimed at the commercial sector. It means private organisations could easily receive direct messages from customers, say. It's not the first tweak Twitter's made to direct messages recently: earlier this year, iOS, Mac and Android apps were updated so that they sync the status of direct messages, and it's been experimenting with focussed news via the medium too.