Microsoft has thrown the switch on a new release of the Preview for Developers and Windows Phone 8.1



From WPCentral:

Starting at 1 PM ET, those who are enrolled in the free Preview for Developers program are receiving build 12400 of the OS, which is a small jump from the previous 12397. Currently, it is not known what changes are brought with the latest release, though it is most likely fixes for stability and other optimizations.

  Microsoft is now releasing build 12400 of the Windows Phone 8.1 OS preview